I just love those people who dress all male then get offended if they get a "sir". Myself, well, my avatar is what I looked like just before going on T, I need to get some more up to date pics taken, 3 months on T have made me a bit more male looking. Right now I just take the "sir" or "ma'am" whatever I'm called graciously and without comment.
But you may get the easily offended types and frankly it's hard to tell because so many are so "metro" so just some suggestions *I* can think of, I'm sure others will weigh in and make this an interesting thread:
Hair - Transmen will try to have a more "male" haircut in many cases.
Bulge - Transmen will often "pack" (wear a "stuffer) in their pants
Friends - what are their friends calling them? Try to overhear that
Testosterone - This will increase body hair, lower voice, etc.
A lot of transmen become "invisible". You just see another dude. The ones that scream to you, "Is that a transman or a butch?" are pre-hormones or haven't been on hormones long. We don't stand out after about 2 years on the "juice".