When did Cat ... ?

Interesting question, and you're right that there are many episodes left in season 1 and all of season 2 which might affect my answer. But I'll take a stab at it anyway. I have always felt that the appearance of both Kara and Supergirl in her office threw Cat off the trail, but only temporarily. Her reporting instincts would keep at least a little bit of suspicion alive. Plus, being Cat, it would be very difficult for her to accept that she had been wrong. Subsequent events would rekindle that belief until eventually she would come to realize that she had been tricked somehow. But by that time she had come to realize how important it was for Kara to keep her secret identity, so she just kept it to herself and did not confront Kara again.

Another possibility is that Cat immediately suspected that the second "Supergirl" was actually just an impostor of some sort (perhaps a member of Kara's family who closely resembled her). Of course, she had no way of knowing that it was actually another alien who had shape-shifting abilities! But with no facts to support it, it was really just a suspicion at that point.
And by the end of the episode (especially after Kara's earlier monologue in Cat's office), Cat had already come to realize - that if in fact Kara was Supergirl - how important it was for Kara to keep her secret identity. So she just decided to play along with the ruse, and did not confront Kara again.

One more thought on this. In the meeting with Cat when Kara quits her job, she doesn't really deny being Supergirl. Instead, she says something like: "You want me to say I'm Supergirl, but I can't do that." It's what a politician might call a "non-denial denial." Although this scene precedes the Kara and Supergirl together scene, Cat would remember it and file it away for later reference.

While I think seeing Kara and Supergirl together in the same room created some serious doubt, I don’t think Cat was 100% convinced. Later episodes likely rekindled Cat’s lingering suspicions. For example, out of the millions of people in National City, why did Bizarro kidnap Kara Danvers?
I think it was “Falling” that eliminated any remaining doubt. By the time of the balcony scene at the end of “Falling,” Cat knows for certain that Supergirl is Kara. When the episode originally aired, I half-expected Cat to actually call her Kiera (or possibly Kara) in that scene.

I think Cat may have hit pause when she saw both Kara and Supergirl, but I don't think she was ever convinced Kara was not Supergirl. Then J'onn was revealed as the shape-shifting Martian Manhunter and she quickly put it all together.

Yes, in the "Falling" episode, J'onn ("Hank Henshaw" ) was revealed to be a shape-shifter in front of all the spectators in downtown National City, as well as everyone at home watching the news on TV.
And then came the "Myriad" episode, in which all of the members of the regular CatCo staff were sitting at their desks, transformed into "zombies", except Kara. Then Supergirl shows up at the CatCo offices - but still no Kara! A dead giveaway, right?

One more thought on this. In the meeting with Cat when Kara quits her job, she doesn't really deny being Supergirl. Instead, she says something like: "You want me to say I'm Supergirl, but I can't do that." It's what a politician might call a "non-denial denial." Although this scene precedes the Kara and Supergirl together scene, Cat would remember it and file it away for later reference.

Yeah great use of nuance in expression there!

Yes, in the "Falling" episode, J'onn ("Hank Henshaw" ) was revealed to be a shape-shifter in front of all the spectators in downtown National City, as well as everyone at home watching the news on TV.
And then came the "Myriad" episode, in which all of the members of the regular CatCo staff were sitting at their desks, transformed into "zombies", except Kara. Then Supergirl shows up at the CatCo offices - but still no Kara! A dead giveaway, right?

I would also add another detail. I might be wrong about this, can't remember whether Cat was present or not. But James and Winn and the girl (forgot her name) jump off the CatCo building. Supergirl saves James and Winn but not the girl. Who is James and Winn best friends with ? Kara Danvers. What would Cat have thought?

I think we have all the bits here so I’ll try to put them together as well as add to them.

-The events in S2E8 solidified any suspicions Cat may have had.

-When J’onn shapeshifts so Cat sees Kara and Supergirl together at the end of S1E9 Cat has no explanation for that and has no choice but to believe her own eyes. I wouldn’t say she’s duped but Cat’s reporter at heart and the fact that she saw Kara and Supergirl together has to overcome her suspicions which aren’t facts.

-In Fallen Kara started acting differently at the same time Supergirl did so perhaps Cat’s earlier suspicions come back but she’s still got no facts to explain what she saw before. Until as others have brought up towards the end of that episode Cat and everyone else saw J’onn shapeshift that’s IMO what would in Cat’s eyes explain how she saw Kara and Supergirl together before. This is the point where I think Cat knows. If Cat didn’t know I don’t think she would’ve been so quick to forgive Supergirl for throwing her off the building and offering advice in their balcony scene at the end.

-In S1E18 Cat playfully hints to Kara that she’s figured out Barry is the Flash. Cat’s started taking a different tactic.

-Cat was with Supergirl and Max then also J’onn, Alex, and Eliza during the events of Myriad and Better Angles. During that time Kara was missing. As I recall in order to snap Alex out of Myriad’s control and stop her daughters from fighting Eliza told Alex something like “you’re not going to hurt me and you’re not going to hurt Kara”. Eliza called Supergirl Kara. We don’t know that Cat watched the fight and heard that but she was nearby.

-In S2E2 Cat made a playful attempt to get Supergirl to tell her her real name.

-Cat was around during the Daxamite invasion at the end of season two where everyone but Kara was present.

Cat made a mistake giving Kara an ultimatum in S1E9 and Kara pushed back, I don’t think Cat expected that. In my headcannon Cat realized her mistake and came to understand why Kara keeps her secret. From S1E18 froward Cat’s attempts to get Kara to reveal her secret identity have been more like a friend than a reporter. I wonder if now especially in light of everything that’s happened with Lena if Kara would be more likely to tell Cat. Cat is not Lena so I don’t think Kara fears a bad reaction but I think she would still be nervous. It would have to be under the right circumstances but I can see Kara reveling herself to Cat. Kara would be nervous but Cat would nonchalantly remind her that she figured it out years ago.

" It would have to be under the right circumstances but I can see Kara reveling herself to Cat. Kara would be nervous but Cat would nonchalantly remind her that she figured it out years ago."

I actually wrote a fan fiction story a few years back in which Kara changes to Supergirl in front of Cat (and a couple of members of the Supergirl Team), in order to stop an ongoing attack from Lex Luthor's henchmen in the parking lot of the CatCo Building. Afterwards, Kara (now dressed as Supergirl) tells Cat that "now you see that you were right all along about me being Supergirl". Cat nonchalantly replies that she already knew that, and had in fact known for a couple of years.

Here's a link to that story on my Facebook page:

https://www.facebook.com/stewart.tick/posts/2115505788580776

Cat made a mistake giving Kara an ultimatum in S1E9 and Kara pushed back, I don’t think Cat expected that. In my headcannon Cat realized her mistake and came to understand why Kara keeps her secret. From S1E18 froward Cat’s attempts to get Kara to reveal her secret identity have been more like a friend than a reporter. I wonder if now especially in light of everything that’s happened with Lena if Kara would be more likely to tell Cat. Cat is not Lena so I don’t think Kara fears a bad reaction but I think she would still be nervous. It would have to be under the right circumstances but I can see Kara reveling herself to Cat. Kara would be nervous but Cat would nonchalantly remind her that she figured it out years ago.

I'm very much inclined to agree with this interpretation.

The "Cat and mouse" game throughout season one was so much fun--and extraordinarily well written and performed, I might add. It works well, whether Cat knows or doesn't, just on different levels, inviting that ambiguity that keeps us talking about it as we approach season 6.

Ok, lets see if I can make an attempt at this. To give credit where it is due, some elements of my analysis will have already been analysed and referenced by others in this thread:

1. In ‘Hostile Takeover’ Cat pieces all the clues together. There is no way to get around Kara’s admission of hearing Dirk Armstrong out of earshot.

2. In ‘Blood Bonds’ Cats interrogation of Kara comes to the point that there is no way out without J’onn’s help. When J’onn as Supergirl leaves, look at Cats expression immediately after J’onn says “Good night Ms Grant, Kara”. Is Cat slightly puzzled by the experience? Does she know something is off?

It is at this point unclear if Cat has been completely deceived into thing Kara is not Supergirl or if she is just going along with it having realised it was a mistake to threaten to fire Kara. Then Cats changing attitude towards Kara (to a purely business and seemingly aggressive relationship) for the next few episodes. Cat blaming Kara’s breakup with Adam for Adam leaving Cat again. Then an altercation with J’onn as Kara (pronounced Carla this time) over whole milk and demoting Kara to assistant number two.

3. In ‘Falling’ there appear to be a few clues that Cat still suspects Kara is Supergirl.

3a. Cat notices a change to Kara’s attitude and shortly afterword a similar change to Supergirl’s attitude when Supergirl throws her off the CatCo building.

3b. An important thing to note is the scene where Cat talks to James and Winn about outing Supergirl as a villain. Cat knows that Kara, James and Winn are a trinity, however she theoretically shouldn’t know of Winn’s connection to Supergirl, yet Cat recognises Winn’s involvement in trying to find a cure.

3c. Then J’onn outs himself to the world as a shape shifter. In theory at this point Cat could realise how she was fooled in ‘Blood Bonds’.

3d. Cat and Supergirl mend their relationship.

4. In ‘Manhunter’, Cat has clearly forgiven Supergirl and later defends Kara against Siobhán Smythe. Interesting that Cat’s attitude changes towards both Supergirl and Kara occur at the same time. It is never a triangle with Cat of love one and hate the other, instead its always the same attitude towards both Kara and Supergirl at the same time. Has Cat attributed Kara’s behaviour to Supergirl’s brain being poisoned?

This is the first point in time where I think Cat actually knows Kara’s secret and can’t be dissuaded but decides to take a different approach to ‘Blood Bonds’.

5. In ‘Worlds Finest’ Cat appears to be on best terms with Kara again. Cat admits to knowing Barry Allen is the Flash and in that moment says to Kara that she “can spot the extraordinary pretending to be a nobody”. Is she teasing Kara here? Is Cat intentionally trying to give Kara signals that she knows and opportunities to open up? because if she knew Barry is the Flash all along, with her intelligence Cat would have to be questioning why of an entire world full of people, Barry is hanging out with Kara, James and Winn. Cat also does not appear to be surprised that all three of them know Barry’s secret. Another point, in order to see Barry off the way it is suggested at the end, for a non-super powered person think how much man hours it would take out of a workday. Yet Cat is OK with it. Cat as someone who in ‘Manhunter’ did not recognise a sick day as a whole day (probably intentionally, expecting Kara to turn up to work), actually lets Kara go. We don’t see the scene but I would expect that if Cat (or any manager in that situation) did not know Kara’s secret, she would be very surprised at Kara’s quick return to work after seeing Barry off. (how does all of this now fit in, post crisis?)

6. In ‘Myriad’ as Non is sending James, Winn and Kelly to their doom Cat is still in the same room to hear Non refer to the Office staff as Supergirl’s friends. Supergirl does mention Kelly by name but only after Cat refers to Kelly first.

7. In ‘Better Angels’ while at the old TV station Cat is in the same room with the Danvers family and witnessing their interactions.

8. In ‘The Adventures of Supergirl’ when Kara finally realises her calling as a reporter and Cat closes her own arc, realising she needs to move on, Cat drops another hint “I can see the hero within you”. Remember way back in ‘Stronger Together’ when Kara and James had a conversation recognising that Cat won’t know that Kara is Supergirl because Cat doesn’t see her assistant and that Clark is “able to hide because the world can’t believe that there is really a hero in their midst”. A lot has happened since then.

9. In ‘The Last Children of Krypton’ I’m not quite sure how to interpret the scene where Cat tries to persuade Supergirl to reveal her real name. Is it that Cat still does not know or that she is giving Kara a final opportunity to open up before Cat leaves?

10. In ‘Resist’ Supergirl tells Cat that her boyfriend is trapped on the Daximite ship then Cat trying to comfort Supergirl talks about her journey to Bhutan.

11. In ‘Nevertheless, She Persisted’ when Cat in reference to the Supergirl vs Rhea fight, asks Kara “Do you think Supergirl is ready for this?”, her tone is almost like and could be interpreted as if she is asking Kara if she is sure she herself is ready. Then the final Cat scene just before the big reveal that Cat knows Kara’s secret, is a repeat of the above ‘Resist’ scene this time with Kara telling Cat about her relationship. Again, another instance where Cat can realise the events around Kara simultaneously match the events around Supergirl. If contrary to what I suggested earlier Cat only realises Kara’s secret during this scene, it could be the moment just before Cat says, “Accept of course you won’t be wearing the Bhutan’s”. Referencing the earlier conversation with Supergirl. The real question is: does this statement also confirm to Kara that Cat knows? And if not then, surly with Kara’s super hearing she heard Cat say “Go get em Supergirl”.

I would love to see this all resolved in a future episode although it may be highly unlikely for various production reasons.

Another thing to note. At the start of Season 4 James establishes that it was Cat who recommended the pairing up Nia Nal with Kara. I wonder if Cat knows that Nia Nal is Dreamer.

"Cat made a mistake giving Kara an ultimatum in S1E9 (the "Blood Brothers" episode) and Kara pushed back, I don’t think Cat expected that..:"

Exactly, it appeared that Cat expected Kara to simply confess. But instead Kara steadfastly declined to admit that she was Supergirl.

" I wouldn’t say she’s duped but Cat’s reporter at heart and the fact that she saw Kara and Supergirl together has to overcome her suspicions which aren’t facts."

Yes, I would say that Cat suspected that the other "Supergirl" was some sort of imposter, but had no facts to support that idea. It was just a suspicion.

"When J’onn as Supergirl leaves, look at Cats expression immediately after J’onn says “Good night Ms Grant, Kara”. Is Cat slightly puzzled by the experience? Does she know something is off?"

Yes, it was if she was thinking, "Now that doesn't sound like the Supergirl I know!"
Also, when "J'onn as Supergirl" first shows up at Cat's office, he says; "I heard you wanted to see me? Something about thinking your assistant was me?"

Again, that doesn't sound like Kara! It's too "brusque and businesslike". But it certainly does sound like "Hank" a.k.a. J'onn. I'd say that contributed to Cat's believing that Kara may have attempted to trick her somehow with a "lookalike double".

And another incident that noone seems to have mentioned yet:

After "J'onn as Supergirl" leaves Cat's office, she says "now that I've seen you and Supergirl together, you look nothing like her."

Now that was disingenuous; they totally look alike (as Cat had acknowledged when Kara took off her glasses in the "Hostile Takeover" episode)! Certainly Cat had decided by then not to confront Kara again about her secret identity, even though she apparently still had her suspicions. In other words, she realized that her earlier actions (giving Kara an ultimatum) had been a mistake.

I think its later in the Fallen episode or probably after where Cat says to Kara something like "I'm not giving up on Supergirl and neither should you

Wow! Lots of really good ideas and theories here. Like everyone else, I've wondered about this same question myself in the past too; but after reading all of the comments below, I realize that I've looked at it from a rather different perspective from most others.
Even the greatest detectives in the world - Sherlock Holmes and Detective Lt. Columbo, to mention just two - were never able to solve any crime with just one or two clues, nor even quickly or on the first try either. That would be like putting two or three pieces of a jigsaw puzzle together and thinking that you know what the whole puzzle looks like. Even with all the pieces, you still have to put them together, and that can take a lot of time.
So why should Cat Grant have been able to solve this puzzle so quickly? After all, it took her two whole months, and several coincidences, to figure it out the first time.
But, as long as we're all having a lot of fun just speculating, and there is no right or wrong, just sharing of many diverse ideas, here is how I always thought it had worked out.

I've always believed that Cat was completely fooled by the charade that Kara and J'onn played on her in Episode 1.09 "Blood Bonds". I got the impression that her arrogant I'm-always-so-right attitude was rattled just enough that she legitimately thought that she really was wrong about Kara and backed off completely.
The later revelation that a shape-shifter existed, in "Manhunter", was seen on TV, but the government would have done everything possible to cover that up, seeing as how it would have led to revealing the existence of the DEO. So just seeing an unidentified shape-shifter on TV would not have been an automatic connection to anything all by itself.
And don't forget that Cat tried to fire Kara twice (not counting her routine weekly threats): Once when she didn't show up for work (because of the Black Mercy), and when she wrote the letter to Adam. If Cat had known that Kara was secretly Supergirl, she might have been more forgiving than that.
In the episodes that followed "Blood Bonds", we also know that Cat was gradually trying to turn herself into a better person, albeit slowly, and this explained why she was gradually becoming more mellow about a number of things, a little bit at a time. Indeed, she even told Supergirl about this during a quiet moment in the middle of the Myriad crisis in Episode 1.19.
I do not believe that Cat ever questioned her decision that Kara wasn't Supergirl after all, or ever seriously reconsidered her suspicions about Kara again, until the Myriad crisis happened.
The catalyst might possibly have come when Supergirl was making her broadcast speech of hope (interesting how that word keeps cropping up, isn't it?) and everyone in the control room was watching, with looks of doubt on their faces - "will this work?"; everyone except for Eliza. She had a look of quiet confidence, like a look that only a mother would recognize on the face of another mother, and Cat noticed that.
And then, when the crisis was over and things calmed down afterwards and started getting back to normal, Cat would have had the time to think about the events of the previous day, without any distractions (like, saving the world):
• With everyone in the city being under Non's mind control, why was Kara the only one in the entire city who didn't show up for work?
• Almost all cities have a command centre of some kind, for dealing with disasters like floods, earthquakes, etc. So why didn't Supergirl go there, instead of going to CatCo?
• Why would Non think of looking for Supergirl at CatCo, of all places, unless he knew that that's where she would be, because she secretly worked there?
• Why would Kara's sister and mother show up at the same studio where Supergirl, Cat, and Max Lord were, even though no one could have known that that's where they were? (And, for what it's worth, there is no reason to believe that Cat was watching what was happening outside the building from inside the building when she and Max were too busy trying to get the old equipment up and working.)
• Most people would probably be kind of in awe over meeting Supergirl in person for the first time; and yet, when Supergirl and Eliza were alone in the studio together (just before she began her broadcast), Eliza was acting very relaxed and familiar with Supergirl. Why?
And, when Cat put all of those coincidences and clues together, and combined them with the Flash's recent appearance and team-up with Supergirl - at the same time that Kara was hanging out with Barry Allen - and then re-considered all of the initial coincidences that had triggered her suspicion of Kara months earlier, I've always believed that that was when Cat figured it out for the second time.
But Cat knew by then that she had made many mistakes with Kara, and Supergirl too, but as she was still trying to reform herself, I wondered if she realized that she hadn't figured out what nearly all of those mistakes were yet. And so, rather than jumping into it with Kara all over again, Cat probably decided to avoid repeating those mistakes by waiting until she had figured out what they were and how to avoid them in the future. That would explain why she didn't confront Kara, and would also explain why she promoted Kara so suddenly as soon as the Myriad crisis was over with (after all, if she had known that Kara was Supergirl before Myriad happened, then why didn't she promote Kara earlier?); she even relented and called her "Kara" too, her first real sign of respect towards Kara who, as Supergirl, had just risked her life to save the entire planet.
In the first two episodes of Season Two, Cat was even dropping hints: "pull up your big-girl pants and own your power", "I can see the hero in you", and some others already mentioned by several writers below. But if she knew Kara's secret before Myriad happened, then why didn't she do those things before then?
Cat never foresaw herself leaving CatCo so soon afterwards. The afore-mentioned conversation between Cat and Supergirl out on the balcony after Cat decided to leave was one of the best moments between the two of them; they had finally been able to put everything in the past behind them and were at last friends. I think that it might also have been the first time that tough-as-nails Cat realized what friendship is really all about. Supergirl even called her 'Cat', and I don't think that Cat wanted to spoil the moment so, rather than trying to trick Supergirl/Kara, Cat was just playfully teasing Supergirl about their names instead.
And that's why I've always believed that Cat was genuinely fooled by Kara and J'onn's charade, and that she didn't figure out Kara's secret again, for the second time, until the Myriad crisis. So, for whatever it's worth, this constitutes my contribution to this very interesting topic.

However, as we all have a few months to go before Season Six starts, I have another idea in this same vein that might help us to wile away some of that time, and which no one has mentioned or seems to have clued in to so far: I think that Cat has also figured out that Clark Kent is Superman.
When Kara met with Cat in James' office at CatCo during the Daxamite crisis (in episode 2.22), she brought Clark with her, and Kara told Cat that "We've just come from an exclusive interview with Supergirl", and said that Supergirl wanted Cat to 'call off the media' so that they wouldn't become collateral damage during what would certainly be a huge battle-royal between Supergirl and Rhea. Cat was reluctant to do this, until Clark used his Kansas-charm to get her to agree.
However, what neither Kara nor Clark knew, or could have suspected, was that Cat already knew that Kara is Supergirl (which even we viewers didn't find out about until the end of that episode). And, because of that, Cat also knew that there was no interview (because you can't interview yourself) and that Kara was just making that up as an excuse to get Cat to agree to the request to 'call off the media'.
However, that also meant that, because Clark was going along with what Kara said about an interview which Cat knew had never happened, the only way he could be a part of that ruse was if he also knew that Kara is Supergirl too. And that would definitely have started Cat wondering about how and why Clark would know such a closely-guarded secret.
After all, as far as Cat knew, the only other time that Clark and Kara had ever met was the previous fall, when Clark was in town to interview Lena Luthor about the 'accident' that happened to the "Venture" hypersonic shuttle-craft. It was Clark's first visit to National City, and Kara escorted him around town. However, even though Clark and James Olsen had been best friends for years, and hadn't seen each other since James had moved out to National City a year before then, Clark spent most of his time with Kara instead of James. Why?
And another coincidence: Superman came to town at the exact same time as Clark came to town; and they both went back to Metropolis at the exact same time too.
Fast-forward back to the Daxamite Crisis: Superman reappeared in National City - to help out his cousin Supergirl - at the exact same time that Clark appeared in town, literally from out of nowhere, and here he was with Kara (who is secretly Supergirl), again. And, as with the year before, both Clark and Superman went back to Metropolis at the same time too.
Further connections and coincidences would logically have become apparent to Cat:
• James is friends with Supergirl, who is Kara; or, best friends with Kara, who is Supergirl;
• Lois and Clark and James were like 'the Three Musketeers' at the Daily Planet;
• James and Clark are best friends;
• James is Superman's "pal";
• Clark is also friends with Superman;
• Lois Lane is friends with Superman;
• Lois and Clark have been living together for a few years now;
• Clark and Superman both have the same 'dreamy' green eyes, as Cat put it.
In spite of the considerable differences in opinions that have been posted on this Forum topic so far, I think that there is one fact that we can all agree on: No one is better than Cat when it comes to spotting coincidences, and I doubt that she could look at all of these coincidences and clues without coming to the only conclusion that would make any sense to her: that Clark Kent is so close to Kara, and knows that she is Supergirl, because he is her cousin, Superman.

This Forum topic is all about when and how Cat figured out "for sure" that Kara is Supergirl; but as long as I was adding my ideas to the excellent mix of suggestions as to when and how that might have happened, adding my suggestions about Clark's secret to the conversation is still in keeping with the question "When did Cat … ?".
Either way; until, or unless, we ever have another meeting between Cat and Kara - which would be the ideal time for Cat to finally explain this mystery of 'exactly when' to Kara, and all of us too - we'll probably never know for sure. So that means that we can all continue to have some fun with this one for probably quite a while to come. And let's face it; this really is a really fun topic.

"I've always believed that Cat was completely fooled by the charade that Kara and J'onn played on her in Episode 1.09 "Blood Bonds". I got the impression that her arrogant I'm-always-so-right attitude was rattled just enough that she legitimately thought that she really was wrong about Kara and backed off completely."

OK - well, that's where we disagree then. As others have said, we were led to believe that Cat started her career as a reporter. So she would have been looking at this like a reporter, and putting facts together to support her conclusion. And she had quite a bit of factual evidence to support her conclusion that Kara = Supergirl. It was certainly too much to logically be "explained away" as nothing more than a series of coincidences. So I never felt that Cat was totally snookered by seeing what appeared to be Kara and Supergirl side by side in her office. Rather, it presented her with a conundrum. There was too much evidence to immediately discount her Kara = Supergirl conclusion. It is my feeling that Cat would rather have immediately questioned whether the "Supergirl" in her office was the "Real McCoy". And as others have pointed out, the dialogue spoken by "J'onn as Supergirl" didn't really sound like what one would expect the "real Supergirl" (Kara) to say in that situation. Rather, it sounded like "Hank's" speech patterns. I know that's pretty subtle, but I do believe that Cat is perceptive enough to catch it. As someone else said, Cat had a "slightly puzzled" look on her face when "J'onn as Supergirl" said "Goodnight, Ms. Grant, Kara". It was as if she was thinking "something's a little off here, that doesn't sound like the Supergirl I know"..
But at that point, Cat had no logical explanation for what she had seen. Then, a bit later on, when she saw "J'onn as Hank" shapeshift into the Martian Manhunter on live TV, I believe it was an "A-ha Moment" (or a "Sherlock Holmes Moment" ) for her. After all, J'onn had to be an alien (humans can't shapeshift), and Supergirl is an alien, so they probably know each other as a part of the "alien community". I believe that that was when Cat realized that she must have seen J'onn shapeshifted into Supergirl (and not "Kara as Supergirl" ) that night in her office. It was the only logical conclusion. And it was the simplest explanation (in accordance with the principle of Occam's Razor).
Anyhow, that's how I see it....

Also bear in mind in Livewire, Cat had seen Kara and J'onn (as Agent Munroe) talking to each other, even joining in with their conversation. Therefore she should recognise J'onn just before he shapeshifted and presumably recall that Kara appeared to know him.

Then, a bit later on, when she saw "J'onn as Hank" shapeshift into the Martian Manhunter on live TV, I believe it was an "A-ha Moment" (or a "Sherlock Holmes Moment" for her. After all, J'onn had to be an alien (humans can't shapeshift), and Supergirl is an alien, so they probably know each other as a part of the "alien community". I believe that that was when Cat realized that she must have seen J'onn shapshifted into Supergirl (and not "Kara as Supergirl" that night in her office. It was the only logical conclusion. And it was the simplest explanation (in accordance with the principle of Occam's Razor).
Anyhow, that's how I see it....

I knew that I was all by myself when I posted my previous viewpoint on when Cat might have figured out 'for real' that Kara was Supergirl.
After re-reading everyone's postings again, I went back and watched all of the Season One scenes that were used to support the opinions expressed so far because, in the interests of fairness, it was important to do this (specifically: Episodes 4, 9, and 11 to 20). I tried to look at things from the point of view that Cat either still knew that Kara was Supergirl, or that she at least suspected it again, rather than having been completely fooled. Also, I paid specific attention to her facial expressions both during and after the exchanges that took place: in particular; when no one was looking, did Cat look as if she was pleased with herself for having pulled off her own charade or pretense, or did she look as if what she had just said or done had no hidden or ulterior motive to it?
However, there is always more than one way to go about solving a mystery, and when I posted my original contribution to this topic, I cautioned that I had used 'a different perspective' to look at it, although I didn't elaborate on what I meant by that. I've debated whether to post this update or not, but you all deserve an explanation to what must seem such a very strange and contradictory position that I've taken, so out of respect for any curiosity that any of you might have, here it is.
The approach I've usually taken towards looking at such possibilities is based on how mystery stories have been presented for decades now and solved in the past; simply put: "Facts in evidence".
Regardless of whether it's a written story, a novel, a movie, or a TV show - or whether it's a murder mystery, a 'who-done-it?', or any other kind of conundrum - when presented as entertainment they traditionally hold to a consistently simple premise: that the detective or person who is going to solve the mystery can not use any evidence that has not already been presented to the reader or viewer; and - just as importantly - vice-versa.
Applying this principle to how Cat figured out Kara's secret, this means that the only evidence that Cat can have used to figure out that mystery is what she herself knows directly, not what we know as viewers. In other words; unless the storytellers deliberately show us Cat actually doing or seeing something herself, then assuming that Cat automatically coulda/shoulda/woulda/must've known or seen anything is entirely presumptuous. So, whether anyone else uses this method or not for figuring things out, that's how I usually do it myself.
And because our individual viewpoints are all purely subjective rather than objective, this isn't about trying to change anyone else's opinions. And you won't have to go back and re-watch anything either; this is just about explaining why I'm so far out of sync with everyone else on this subject (sometime called "food for thought".
But as for the actual episodes themselves, here's what I noted on watching them again.

A few people have attached some significance to how Cat had met J'onn J'onzz once before; when he showed up at CatCo after Leslie Willis's attack on Cat and Kara in Episode 1.04 "Livewire" and that Cat saw Kara talking with J'onn and therefore 'the reporter in her' must have suspiciously assumed that they knew each other.
But don't forget; at that time, even we viewers didn't know yet that Hank Henshaw was anyone other than the jerk who ran the DEO, and even we didn't find out the truth about his glowing red eyes ourselves until Episode 1.07. So, just meeting 'FBI agent Monroe', as Henshaw introduced himself (and "Agent Mulder" as Cat sardonically called him) wasn't anything that could have tipped Cat off to him being anything other than who or what he appeared to be. And please forgive me if I'm really stretching things here, but wouldn't you think that any detective or investigator, upon showing up at the scene of a crime, would naturally want to question the only two witnesses; namely Ms. Grant and Ms. Danvers? So why would there be anything about that brief encounter that would give Cat any reason at all to be suspicious of seeing 'Ker-rah' talking to the FBI Agent? Logically, there wouldn't.
Now jump forward to the meeting between Cat, Supergirl, and Kara in Cat's office in "Blood Bonds". Some contributors have suggested that Supergirl supposedly behaved much more like J'onn J'onzz than like the real Supergirl, and that 'the reporter' in Cat would have immediately picked up on that. However, (facts in evidence); at that time, Alex and Kara were the only two people on Earth who knew anything about J'onn's secret. No one else anywhere, not even the staff at the DEO, knew anything about Hank Henshaw being J'onn J'onzz, or about Martians or shapeshifters. WE viewers knew by then, but Cat couldn't possibly have known anything about J'onn, or shapeshifters, and that's why she didn't have any reason at all to suspect that the person who dropped by her office that night was anyone other than the real Supergirl (at least not on that night). And, for whatever it's worth, even we viewers didn't know that it wasn't really Supergirl until Kara walked in a few moments later. And Cat knew that Kara was an only-child, because in the "Livewire" episode Kara told Cat that her parents had 'died in a fire' when Kara was 13; Kara didn't say anything about having any other siblings, just her foster mom and sister, so it couldn't have been another family member who was covering for Kara that night.
And remember; the relationship between Cat and Supergirl was still very acrimonious at that point in time. Cat had written a scathing interview/editorial after her first encounter with Supergirl and, except for the two of them teaming up to stop Livewire, and Supergirl later thanking Cat after the earthquake in Episode 1.07, there was nothing else very friendly going on between them at that time. In fact, when Supergirl showed up in Cat's office that night, Cat's opening caustic comment was that Supergirl's suit (the three primary colours of red, yellow, and blue) "looks like a colour wheel threw up"; hardly a very complimentary way to start off a conversation. In watching this scene again I got the impression that, given the animosity that already existed between Cat and the real Supergirl at that time, and the reality that Cat actually knew very little about Supergirl herself at that point, Supergirl's reactions really weren't out of line with what Super/Kara's own reactions would have been; she ignored the sarcastic insult and got right down to business.
Also, in looking carefully at Cat's expressions and body language both during and after the meeting, I just can't see anything that looks to me as if Cat was thinking to herself "You thought you could fool me, but I'm fooling you instead", or anything like that. I know that everyone else commenting on this topic sees it otherwise, but I'm just not reading anything more into what I see than just what it appears to be; that the ruse worked and Cat was tricked. In fact, she was so embarrassed at being wrong (which she reluctantly admitted) that she even asked Kara not to mention the incident to anyone else, and I didn't get the impression that she was faking that.
This doesn't mean that she couldn't have had second thoughts later on, but IMO she just looks to have completely accepted that she was wrong about Kara at that time.
Some people have suggested that Cat might have been fooled, but figured it out again later and decided not to say anything to Kara because Cat realized by then how important it was to Kara to keep the secret to herself. I am in complete agreement with everyone on this belief; my only disagreement is about when that happened.
As another example: If Cat supposedly wasn't fooled and still believed that Kara was Supergirl, even after seeing Kara and Supergirl together in her office, then her understanding of Kara's desire to protect her 'super secret' is the antithesis of Cat's actions towards Kara over the next few months/episodes, including the second time that J'onn covered for Kara; when she was incapacitated by the Black Mercy in Episode 1.13. J'onn's performance in covering for Kara was as disastrous within the storyline as it was hilarious to see, entirely because of Melissa's impressive performance of pretending to be J'onn ineptly pretending to be Kara; it was wonderful to watch, and I would loved to have seen the 'gag reel' for those scenes. But (facts in evidence); this was still quite some time before Cat or anyone else in National City (or even at the DEO) found out anything about either J'onn J'onzz or Martians or shapeshifters; so regardless of how very strange Kara's actions were, or of 'the reporter in her', Cat realistically didn't have any logical reason to suspect that this was an impostor rather than the real Kara. In re-watching these scenes, I saw nothing that suggested to me that Cat had figured out that Kara was in trouble with something 'super' and that she should cut Kara's stunt-double some slack because of that. Quite the opposite; she threatened to fire Kara, and then later became so disgusted with Kara's quirky behaviour that she told her that “I can not stomach looking at you any longer” and angrily sent her home; something that I seriously doubt whether Cat would have done if she had suspected that it might have had something to do with Kara's secret about being Supergirl.
Kara's change in behaviour in "Falling" might have been interpreted as nothing more mysterious than having to fight for her job against the scheming Siobhan Smythe. And as for Cat defending Kara over Siobhan; honestly, Siobhan's consistently conniving and manipulative behaviour was her own undoing, and we all know how Cat detests disloyalty; so I didn't see it as Cat defending "assistant number two" nearly so much as her simply dealing with a detestable employee in her own inimitable way.
Another point which almost everyone has focused very heavily on is J'onn's revelation as the Martian Manhunter near the end of "Falling". True enough, WE got to see what happened on our TVs, but Cat wasn't there to see it happen herself and, even more importantly, nor were any members of the press there to televise what was happening on live TV (and I replayed this scene 4 or 5 times). And because we didn't seen the press there, there's no way that the event was being televised for Cat to see on a TV somewhere else. "Facts in evidence" therefore says that she didn't witness it happen, and therefore she did not have personal knowledge of it. And I also noticed that early into the following "Manhunter" episode we even saw the press announcing on the TV news that they had no information of the 'mysterious unknown alien', and absolutely nothing at all was said about either Martians or shapeshifters at that time.
Admittedly, that doesn't mean that Cat didn't find out about it afterwards. There were civilians present, and they were shown recording what was going on with their cell phones. But given that it was at night, with less light than in the daytime, and that the phone-videos taken from a distance would not have been of press-camera quality, there's no reason to assume that the pictures were clear enough that, even if Cat might have seen the videos later on social media, she would have looked at them and instantly thought "Oh, a shapeshifter; that obviously must be the exact same person who helped Kara try to fool me months ago". Is that possible, yes, absolutely, but how likely is it? After all, if that information was 'out there' for everyone to see and know about, then why didn't the TV news announce that information?
Cat's forgiveness of Supergirl in that balcony scene probably had more to do with Supergirl going there to apologize to Cat - which she did - and Cat wanting to mend her relationship with Supergirl than anything to do with Kara. Admittedly, Cat's facial expressions could easily be interpreted in many different ways; but, again, that's where the fun of speculating comes in.
And don't forget; near the beginning of the following episode Cat was calling for 'Ker-rah' as if nothing was different, and when Winn reminded Cat that Kara had phoned in sick, Cat flippantly commented that "that was at 9 a.m.". Again, hardly supportive of someone who she might have surreptitiously been trying to help out, especially considering her forgiveness of Supergirl in the balcony scene in the previous episode.
While re-watching the "World's Finest" episode again, there could be a lot credibility to the suggestion by some that Cat started to suspect Kara again during Barry Allen's visit, and it's a very good point. But that was just days before Myriad happened, and it is still far removed from the meeting in Cat's office in "Blood Bonds" months before.
As for Myriad; when J'onn showed up at the old television station in "Better Angels" it was only the second time that Cat and J'onn had ever met in person, but she absolutely would have remembered 'Agent Mulder' from the "Livewire" event several months ago. But by then, it would have simply been added to all of the many other clues that she was already noticing during the Myriad crisis, and adding them to the other coincidences over the past few months.
At any rate, right or wrong, I believe that Cat fully knew Kara's secret by the end of Season One (again; the sudden and unexplained promotion, accompanied by a complete 180-degree change in attitude by Cat towards Kara), and therefore nothing that happened during Season Two had any affect on her beliefs towards Kara being Supergirl by that time.

So, for whatever it might be worth, that's how I looked at everything; not from the perspective of what we viewers knew, but from what Cat knew, and/or what she couldn't possibly have known about. In looking over everyone else's opinions posted so far, it's clear that I'm pretty much alone in my ideas about what might have happened and when and how, but I don’t mind; I've been all alone before, so that not a new experience.
I would love to see Cat make a return appearance on the show sometime (and maybe even some "Special Guest Star" appearances from time to time), and if that ever happens, then it's almost incumbent on her to let Kara (and all of us) know what really happened on her end. And if that happens, then it's quite possible that everyone else might be right in one way or another, in whole or in part, and I might find out that I'm totally off-base and completely wrong - and I still won't mind.
This topic certainly doesn't have anywhere near the importance of the 'saving Lena's soul' theme that was central to Season Five, and that's why I think of this as a 'fun topic'. As I said above, this is just about sharing ideas, so no one has to change their opinions about anything. But even if no one agrees with my perspectives, or with my relying on 'facts in evidence' for my viewpoints, I hope that this would just simply illustrate how different methods of solving puzzles can easily result in arriving at opinions that are so different from everyone else's. And if you understand that, then this posting has served its purpose. Thank you for reading it.

As an avid reader of mystery novels I appreciate the perspective of “facts as evidence” and I agree that the majority of authors try to follow it to a great extent in books. IMO it’s less strictly followed in a visual medium like film. For me for a book to be a successful true mystery I as the reader don’t have to get all the clues at exactly the same time (but timing does matter) as the protagonist but we need to have the same information before the crime is solved. It bothers me a lot when the protagonist solves the crime with information that I as the reader don’t have. It also bothers me a lot if the author makes the mistake of having the protagonist solve the crime with information that I as the reader have but the protagonist doesn’t. I also don’t like it when I as the reader get a clue so far ahead of the protagonist that I figure out who done it with many pages left to read, that’s where the timing comes in.

This was a fun exercise and thinking about it using “facts as evidence” is an interesting perspective which I hadn’t thought of. Certainly from Cat’s perspective she was trying to solve the mystery of Supergirl’s identity. I guess I didn’t think of things from that perspective because I as an audience member wasn’t trying to solve that mystery along with Cat. The writers and actors may have had fun dropping or not dropping clues for Cat in order to facilitate discussions like this one. Certainly I can imagine Melissa and Calista playing with different facial expressions which could’ve changed things.

Richard,

Here are my reponses to some of your points. There's always another way of looking at it. I can't speak for anyone else, of course, but this is how I saw it:

"Some contributors have suggested that Supergirl supposedly behaved much more like J'onn J'onzz than like the real Supergirl, and that 'the reporter' in Cat would have immediately picked up on that."

Umm... that's not really what I was trying to say there. Of course, Cat didn't know anything about J'onn. My point was that "J'onn as Supergirl" was acting (speaking, really) differently that what we had seen previously from the real Supergirl. To me, the expression on her face when J'onn left seemed somewhat puzzled, as if all the pieces weren't quite fitting together.

" I just can't see anything that looks to me as if Cat was thinking to herself "You thought you could fool me, but I'm fooling you instead"

Neither can I - nothing that conclusive. More like "something's a little off here, this doesn't seem quite right somehow - it doesn't really add up.." I don't think that Cat knew that Kara = Supergirl at that point. She was conflicted about the question, and hadn't yet decided one way or the other.

"And Cat knew that Kara was an only-child, because in the "Livewire" episode Kara told Cat that her parents had 'died in a fire' when Kara was 13; Kara didn't say anything about having any other siblings, just her foster mom and sister, so it couldn't have been another family member who was covering for Kara that night."

Yes, if you're limiting it to immediate family members. But Kara could have also had aunts, uncles or cousins that Cat didn't know about. Also, Cat knew that Supergirl was Superman's cousin. So maybe there was another super-powered cousin out there.

""Oh, a shapeshifter; that obviously must be the exact same person who helped Kara try to fool me months ago". Is that possible, yes, absolutely, but how likely is it? After all, if that information was 'out there' for everyone to see and know about, then why didn't the TV news announce that information?"

Good point about no press or TV cameras being present when "Hank" shapeshifted into the Martian Manhunter. I hadn't noticed that (and assumed there would have been, at such a major event in National City). But Cat would almost certainly heard about what happened from people who were there at the time, and actually saw it. Since there wasn't any clear video footage, and it was "just talk", I wouldn't expect TV news announcers to put it on the air.(They go with the facts, not just hearsay.) But if Cat heard it from people she knew and trusted, then she would certainly be inclined to believe that the shapeshifting did in fact occur..

Yes, I know this is "assuming facts not in evidence". But it all certainly seems reasonable and more than likely to me. And in the final analysis, we're talking about something that's completely fictional. So there are no "right" or "wrong" answers, and everyone has their own opinion.

Thank you, Brierrose and Stewart Tick, for your responses to my last posting. And my apologies for my delayed acknowledgement; I don't visit the site regularly - more like once a week, and sometimes even longer than that between visits. Everyone needs to be able to express themselves on the Forum, and although I get lengthy at times, it's never about trying to override anyone else; just about adding my thoughts to the many other good ones, as you have done since my last posting.
And I agree with most your assessments, because you were actually recognizing some of the same points that I was trying to make; principally, that there are always different ways of looking at things. And for reasons that may not always be readily apparent.
Years ago, I lived in a townhouse complex and was elected (eight different times) to serve on the Strata Council during my fourteen years there; and also had the chance to serve on a number of different committees in my unionized workplace too (and was elected to be Chairperson of two of them). Part of our responsibilities were to look at every issue that we had to deal with from more than just our own point of view. It was essential to consider what 'the other guys' would be putting forward as their position whenever we met with 'them', especially as we expected that they would definitely have considered all possible options too, including those not of their liking. Consequently, we had to play "the devil's advocate" with each other while considering what our best position would be on each and every issue, and there are many ways to go about doing this.
At one extreme end of a 'sliding scale' is to decide ahead of time what you want something to be, and then to look for anything that will support your preconceived beliefs, however presumptuous, or regardless of any facts. At the other extreme end is to look at only the facts, while disregarding any 'possibilities', and hope that the facts alone will eventually lead you to a conclusion, or at least a theory; a presumption which has its own set of perils.
In reality, we have all probably used both methods ourselves at one time or another; and are more likely to use a blending of both methods, depending on what it is that we're dealing with. The trick is how far along that 'sliding scale' you want to go in either direction with any particular issue.
But in the end, there was always someone else we had to answer to: the dozens of other owners, or the hundreds of members in our workplace and union. And when you were making your presentation of what you were going to be doing for them, they were very rarely interested in conjecture, assumptions, or speculation; the only thing that they usually wanted to hear were the facts that you had used to arrive at whatever conclusion you were going to use on their behalf.
I don't live at the town house complex anymore, and haven't run for any workplace committees for more than three years now. And, naturally, The Forum here doesn't carry the crucial importance of those kinds of issues from years ago in those settings, with so many trusting people depending on you to 'get it right' for them. But old habits die hard, and the little ole devil in me sometimes just has to have some fun with different opinions, and that's all that goes on with me when I'm adding my ideas to any topic here. And, as with the council or committees from years ago, it's not about being argumentative or confrontational at all - just about seeing different points of view and hopefully getting others to appreciate them as well, even if they don't agree with them. If you get challenged, and your ideas don't hold up, then maybe they weren't carefully thought-through in the first place; but if you do re-visit your ideas, and they do hold up, then you can continue to justify your viewpoints with even greater confidence, which is good for you.
As both of you quite correctly pointed out, this is fiction and fantasy, not real life. No one is going to believe that an alien could really come to Earth and fly around, or that a normal human could be struck by lightning and become a speedster. But while we accept those ideas as the premise for these shows, does that mean that we should also attribute non-human abilities to the 'normal' people on the shows just because it's a fantasy (rhetorical question)? And so, in order to keep myself personally grounded in some semblance of reality, I usually don't accept that people who have no proven knowledge of something can somehow have that knowledge anyway, just because it can be tempting for me to think that they could have or should have seen or known something.
After all, if anyone else who has met Kara and Supergirl could do what Cat supposedly did, then dozens of other people at CatCo would supposedly also know that Kara is Supergirl, or at least suspect that; and many people at CatCo would also have figured out that Nia is 'Dreamer'; and countless people in Star City who met Oliver Queen during the first five seasons of that show 'must have' figured out that he was the 'Green Arrow'; and just as many people who met Barry Allen 'must have' figured out that he was 'The Flash'; and so on.
Speaking only for myself, I have to draw the line somewhere, and that is why 'facts in evidence' is what I mostly rely upon. I do try to consider all of the possibilities along the way, but when 'push comes to shove', the facts that are clearly evident usually take precedence over assumptions that are just speculation or wishful thinking on my part. It doesn't matter if it is just me and no one else who does this, because I could be just as wrong as anyone else, and very well might be if Cat ever reveals what she knows.
Based on your responses, I get the impression that both of you understand that, and I thank you for expressing your ideas on that, even in contrast to mine.
And Brierrose - good of you to notice my slip-up: I didn't mean to imply that we were trying to solve a 'who-dun-it' along with Cat, but after the meeting she had with Supergirl and Kara that night, the question of whether she was fooled or not - and if she was, then when did she start to suspect Kara again - was the reason for the question that started this topic. However, while I was using the approach that Cat couldn't figure out Kara's secret without clues that we actually saw her have undeniably clear access to, I didn't make this clear at first; so good for you on spotting that oversight on my part - you're a better detective than I am.
And, as I have said before, this isn't nearly as important an issue as some others have been, but simply a fun question that has many ways of looking at it. And with Season Six being the last year of this wonderful show, it is almost incumbent on the show-runners to tie up this loose end by having Cat return some day, to set the record strait with Kara, and let the rest of us know what was going on with her. Whatever we find out, it shouldn't change things because, as has already been said, it's not about being right or wrong, but about the fact (no pun intended) that we've all had fun sharing our various ideas on this topic.