Look. I was being flip, but also, ha ha only serious.
If I am writing AOS Kirk, you should be able to tell. If I'm writing AOS Spock, or Uhura, or McCoy, or Scotty, you should be able to tell. (Chekov and Sulu are admittedly more difficult due to smaller amounts of screentime in both timelines). I should be able to drop a scene on you, unlabeled, and have you know who these characters are.
In the officer's mess that evening, Jim flung himself into a chair between Spock, who was reading a paper draft from Astrophysics, and McCoy, who was reading a novel on a padd. Sarek took a much more sedate seat across the table. "So," Jim said, taking an obnoxious crunch from an apple, one arm across the back of Spock's chair. "Ambassador. Want to tell me why you threatened the Romulans earlier?"
If you know both timelines' Kirks, this should be easy to answer. This scene technically could fit into either timeline -- Sarek could easily be on the Enterprise for a diplomatic mission involving the Romulans in either one. No one's eye color comes up. No one, aside from Kirk, is doing anything out of place for either timeline....but Kirk is. He is wildly out of character for one timeline, and not at all out of character for the other, even though both Kirks would say what he is saying here. It is in how he says it, what he does surrounding that question. You should be able to tell, and in fact, if you know both canons, I will be very very surprised if you can't.
These characters overlap, which means that for varying lengths of story, they might be so similar that it's hard to tell -- but they're not the same. Kirk is by far the most distinct, but all the others have noticeable differences that make their interpersonal dynamics different, their word choices different, etc. AOS Spock is more volatile than TOS Spock, probably because of (a) trauma and (b) being almost a decade younger. AOS McCoy is less racist about Vulcans in general and is less pissed off at Spock in particular. AOS Uhura has a much bigger chip on her shoulder and is hostile to Kirk most of the time; TOS Uhura and Kirk are solidly mutual friends.
I read (and write) in both timelines and I'm dead serious when I say that if something "works for both", it doesn't. It can't. Tag it however you like, I guess, but I'd strongly suggest you think about who you intended it to be when you wrote it and tag it that way, because if you did even a quarter-assed job of characterization it won't read like the other one, and your readers will be able to tell.
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