Well you can't know for sure. Here are some possible signs:
* If they say "dude" or "man" or "bro" when referring to you, or use a generally collegial tone.
* Some people stare intently, because something is triggered but they're not sure which.
* I get judgmental scowls or self-satisfied smirks (because they figured it out).
* When you smile at women and they don't smile back. Might just be because they're grumpy, of course.
* When you flirt with men and they don't flirt back. Might just be because they're not interested, or gay.
But in the 21st century, when we routinely meet people who are out of the ordinary, different cultures, disabilities, physical charateristics, etc., nearly everyone at this point has cultivated enough of a studied indifference that you're not likely to know.