Last weekend, I went to an event where I was introduced to a family member of the host. Now, I happen to have known for some time that this young person is a trans man but I'd never seen them, not even in pictures.
I initially saw the person from some distance about 5'5 (maximum), short, spiky hair. Smooth face (don't think they shave) flat chested (binding/masectomy idk). Masculine coded clothing (especially given the event). But, my first thought upon seeing them and basically guessing who they are was that they obviously look female. You can tell they are female. If not, maybe they look like a boy of around 13 or 14.
Then we were formally introduced. I think i was visibly shocked by their voice. It was so deep. Deeper than any man in my family. Like, the bass was wild. It would be a deep voice for a man.
I wondered if they were putting it on, but they seem to speak like that all the time. Can hormones do that? I don't know.
Anyway, my point is that with this person, if you were more unsure than me about their sex, their voice wouldn't lead you to a correct answer.