My walk always gives it away. Apparently I walk like a runway model.
Other than that, I can normally tell when:
They're music taste is female artists only.
They like nickelback.
I'm kidding
Sometimes they way they smile.
Most of the time if they dress nicer + the gay accent.
If they have mostly girls as friends.
Don't like sports
I'm sorry if this is offensive, but if they're on the swim team...