I actually have a hard time "clocking" trans people in public. There's been a couple of times where I've seen someone that I thought was trans, but couldn't tell for sure.
It's really easy to "clock" someone and nitpick them to death when they're in a trans support group or social group and therefore you can know for sure that they're trans. Their "male" features or "female" features become way more obvious, and it's easy to judge them for it. But out in the wide world, where you can never know for sure if someone is trans or not, it's really hard to say yes or no definitively. Especially if they have a good voice. (That is seriously the ONLY dead giveaway.)
The first time I met someone who I believed was trans, it was at a poker table while I was working in Atlantic City. She had a lot of telltale "trans" signs such as a bigger head, high square hairline with thin hair, wide shoulders, narrow hips, brow bossing, a male-ish chin, and big hands. But here's what made me doubt. Her voice was perfect. And if she was trans, then she must have been LONG post-transition, because she wasn't nervous or paranoid at all, she just sat and blended right in with the two other women at the table. So again, I thought she was trans, but I don't know. Maybe? I couldn't tell for sure because she fit in so well. I kept my mouth shut. That's what I do every time I think I see a trans person in public. Because when they blend in so well, odds are that they're stealth and that "clocking them" would be a very unwelcome experience.
Second time I met someone that I thought was trans in public, it was at a job interview. And there was a guy there with a very small feminine build, and a feminine young-looking face. And when he spoke it kinda sounded like a lot of trans men sound, where although the pitch is perfectly in the male range, the resonance and inflection are still a bit androgynous. But as usual, there was no way for me to tell for sure. His facial features were male enough to be cis, still a bit of a masculine chin and a bit of a thick brow, his voice, although FtMish still could have been within the cis range, his build was small but not unquestionably small, so again, ultimately I had no way of knowing for sure.
Another time, I was driving home from a fireworks display, and I was having "one of those nights" where I was looking at the women on the sidewalk as I drove by, trying to convince myself that my shoulders weren't too big. There was one woman who I looked at and saw that she had a more male-ish build, and I actually told myself "see? That woman has bigger shoulders, and that doesn't stop her from looking completely female." I watched her for a moment, and then about 10 seconds later it occurred to me "Wait a minute! That's Anne Marie." (My trans friend.) So once again, there was someone with features that I usually nitpick to death because I know she's trans. But until I realized it was her, I assumed she was just a cis-woman with a more androgynous build.
Again... it's hard unless you know for sure that someone is trans. Once you can definitively say "yes, this person is trans," then their birth sex's features are obvious and you can nitpick them to death, but without knowing for sure, you can only guess.
There's been many trans people at my support group who I thought would never pass visually just by looking at them, who in fact never get questioned in real life.
This is why I'm so adamantly against those who say that trans women "never" pass. BS. Even I have a hard time clocking people even though I know exactly what to look for. (My therapist even laughed at me when I told her about 'brow bossing' and all of these other trans signifiers that I was worried that I had, saying "Carrie, people aren't that observant. I think it's fascinating that you know all of these things, but I never knew about them until you taught me about it. Everyone else just takes people at face value.")
