I'm usually good at identifying trans men because I am one and the "signs" are easier to recognize when you have them, yourself.
Height, facial bone structure (look for high cheek bones and rounder jaw line), voice, skin texture and muscle mass (look at upper arms) are indications but the last 3 only apply when they're not on T. Also check if you can see any ridges of a binder through the back of their shirt.
I'm not saying that it matters, though. Whether or not he's trans is kinda irrelevant since he's a stranger. And do not ask him; I would be offended if I was ever asked that.