Well, obviously every human host/form keeps quiet about the fact that they are Ultras. (Why do they? Why can't they tell people?) My question is more about other people. Do they remain clueless until the finale (or even beyond)? Or do they figure it out somehow?
Just going to do this with new series. I don't remember/still haven't seen some series before Tiga (and I thought, apart from very few exceptions, they usually didn't guess about the hero back then either). This is going to spoil the series mentioned, so if you don't want to know, you really should not proceed.
Tiga:
I'm pretty much sure that only Iruma and Rena know before the Finale. I don't think the male team members are less observant, but perhaps they just didn't watch Daigo as much as the women do. (And, personal opinion only, I think Daigo at least attempted at, you know, keeping his secret identity a secret. He didn't usually transform in a place he could easily be caught.)
If I'm brave, I might say Iruma might have suspicions as early as Episode 3, but seem to know for sure by Episode 25. With Rena, I'm sure she knows in Episode 39 when she asks for Tiga to appear but looks straight at Daigo. She probably has her suspicions over time, but Episode 35 might be the one that makes her sure (an alien possessed her and talked to Daigo as if he was Tiga. Which, to be fair, he was).
Dyna:
It seems to me that no one has any clue except perhaps Hibiki. I wonder if they simply never thought about it, or did but found the idea so impossible. (Seriously, Asuka crashed like how many planes and they never wondered how he survived?) Ryo found out (or more like saw a recording of Asuka's detransformation) in Episode 49.
(Though every alien seems to know that Asuka is Dyna. Why is that? Is Ultra host easy to recognize? That doesn't explain other series where the host can keep his identity secret from some aliens at least.)
Gaia:
Ishimuro knows, probably from very close to the beginning, too. Gamu knows that he knows by mid-series, but they never really talk about it. (Though I seem to remember Ishimuro thinking 'Gamu' several times over the second half of the series whenever Gaia appeared. I really can't get over the fact that Ishimuro really really likes Gamu, almost unnaturally so for someone in his position (to care so much for a subordinate, I mean). Why? Because he knows that Gamu is Gaia? Or perhaps that isn't the reason, but I still believe that if not for Ishimuro, Gamu would have a much more difficult time balancing being in XIG and going to fight as Gaia. That's basically what made me guess that Ishimuro probably knew, but I didn't expect they would confirm that by mid-series.)
The Lizard team from GUARD seem to know about Fujimiya (the rest just seemed to think he could summon the blue Ultraman. Weird (although the summoning idea wasn't unheard of - just look at Ultraman Cosmos), but they probably never thought that Ultraman could fuse/bond with a human).
Most everyone in XIG seems to know by the few episodes before the last.
And this is probably the only Ultra series that I know of that have Ultras' detransformation into humans being broadcasted live on TV. And, since there is no memory erasing device in this series, everyone who watched TV knows who Gaia and Agul are.
Cosmos:
I used to think that Team EYES don't seem observant when Musashi doesn't even try to conceal his secret identity all that much, but well, I guess I should have more faith in them. Fubuki could have known since as early as Episode 15, but, as he said, he was afraid to admit that to himself. (I still think he was sure by Episode 37. Honestly, when he denied that meeting Musashi had changed his outlook, he sounded like he was protesting a bit too much.) Ayano probably had a strong suspicion back in the mid-series upgrade episodes (29-30) but seemed to know for certain by Episode 36. I don't know when Hiura noticed, but surely sometime before the finale (might be back when Choas Header wanted to possess Musashi for the first time or even when he asked why Musashi needed more power (Episode 28)). I think only Doigaki doesn't seem to have any clue. (I mean, even that sleazy doctor knew (on his second appearance), and he appeared like only 5-6 times in the whole series. I found it kind of funny that he tried to keep that from Ayano in the final episode. Kind of wish Musashi would just say, 'stop it, you both know anyway'.)
Nexus:
Open secret all around. Komon found them all out anyway.
Hmm, but still, the Night Raiders didn't know about Himeya until Episode 18, at least (because Komon didn't tell them until then, and Nagi just thought he was a beast and wanted to kill him herself).
As for Ren, Komon purposedly sought him out after the first fight. And it seemed a few people in TLT had become aware of Ren being a dunamist over time (and I loved that they all shut up about it and at most let it be open secret that they never really talked about directly). Mizuo found out in Episode 34, and it's a good thing that she did for Ren was taken away to do experiment on by the very next episode.
Nagi never got a chance to conceal anything as she was too angry and consumed with hatred toward her parents' killer.
I found it so funny that Wakura could totally tell that the Nexus who saved him was Komon. In a way, I'm kind of glad that they didn't do more with Komon and Nagi being dunamists - I mean, how could they ever hope to go fight as Nexus while still being Night Raiders (a problem not usually shared by Ultraman hosts from other series)?
Max:
No one got a clue, it seemed, though I did have some hope for Hijikata and Mizuki. (I honestly thought Mizuki was covering up for Kaito for some time already. Well, maybe it looked that way because I wanted her to be more observant. Then again, when the truth came out, she said something like in a way she probably did realize it all along that Kaito was Max, so perhaps I wasn't entirely wrong in my interpretation. As for Hijikata, perhaps deep down he knew but just didn't want to admit it to himself. You know, because if he did, he might have to feel more (responsible, anxious, etc.) for Kaito. Not knowing that your subordinate is secretly Ultraman is surely less of a burden.)
Aliens usually knew who Kaito was. But sometimes it's quite ambiguous whether they knew or not. (That Alien Sram definitely learned, but I'm not sure it knew about Kaito in his own series. And speaking of that alien, surely I missed his appearance (and feud) with Max elsewhere because, unless I misremember something, one episode is truly not enough for it to carry such a grudge.)
Mebius:
Everyone in GUYS finds out mid-way through the series. No one (apart from Sakomizu and Misaki, who were in from the beginning) seemed to have any idea before that (though I sort of toyed with the idea of having Teppei know). In any case, they (other than Toriyama and Maru) all know by mid-series, which leads to some interesting stories later on. Still the only traditional Ultra series that makes the reveal mid-series, and I even sort of guessed it (not how it happened, though, which was way better than anything I could have come up with).
Guest characters are usually proficient in ferreting out Mirai's secret. I don't remember if every alien identifies Mirai as Mebius, but enough does.
UltraSeven X:
At the end, like every other series. This series actually doesn't need a reveal at all, but I guess they need one to fulfill some hidden requirement for an Ultra series.
Ginga:
Misuzu knows from the beginning because she is with Hikaru when he gains power. Chigusa knows by Episode 4; Kenta, Episode 5. By the end, everyone in that little town seems to know (or maybe they don't, but I mean those core people in the second half of the series, at least). Another series where I'd guessed the hero's friends had to be in on the secret.
Ginga S:
This one goes traditional in that no one knows other than the other Ultraman and Hikaru's friends. But I like that the captain actually figures it out near the end.
X:
This is harking back to the classic in that no one really knows. Well, except the Fantom alien (Dr. Garuman?) who serves as a doctor/scientist for Xio (in Episode 5, I think). I thought it was a bit of a missed opportunity that they didn't think harder when they saw that all the visiting Ultras (except Zero) had human forms. (I mean, Asuna did sort of ask the Ginga S character about Ultraman being human, after all.)
Orb:
Naomi tells Gai in the final episode that she has known from the beginning and I believe her. I mean, I wasn't exactly paying attention but even I was sure she knows (and probably for a while) by the time I got to Episode 17. (And please, don't tell me that she says only that he comes from the stars when all evidence points to her knowing about him being Orb. She always knew where to find Gai whenever Orb got knocked out of his transformation.) Jetta and Shin had their suspicion once early in the series but dismissed it (I don't know why they gave up this line of thinking so quickly? They even found his picture from a hundred years ago). So, apparently, they didn't even find out about Gai in the series itself.
Geed:
I find this series so refreshing on the secret identity thing. Laiha found out in Episode 2. Leito/Zero by Episode 3 (but confronted Riku in Episode 4). Moa saw Riku transform in Episode 10. And Moa's boss, Zena, by Episode 14 or 15. Coordinating attacks are totally possible because everyone can talk and plan together.
(Though I'm not sure/don't remember when/how the AIB found out about Zero/Leito? They (sort of) kidnapped Leito in Episode 11, so they knew before that.)
R/B:
Hmm. Makoto Aizen probably watched the brothers from the very beginning (didn't help that it was their mom who stole the gyros from the company). Asahi saw them detransform in Episode 12. Saki Mitsurugi probably knows because the brothers are terrible secret keepers. Their dad knew in Episode 22 because their mom spilled the beans. Did I miss anyone else?
Taiga:
They (EGIS) said they knew about Hiroyuki being Taiga but kept quiet. I guess it made some sort of sense when thinking back on events. (The midway story arc came to mind.) And I always felt something was off with the team. They never felt the need to rein in Hiro whenever he ran off. (And I think they always know where to find him.) There is one episode that sort of threw me off, but I guess that was more Homare messing with Hiro.
Z:
Hebikura because ah. Yuka found out with the rest in the final episode. Bako-san I'm not so sure of, but apparently he knows by Episode 24. As for Yoko, I'll stand by my opinion that she found out about Haruki sometime before mid-series.