Ask him what he thinks of judy garland, if he says, "who?", then he's not gay.
Let him know that YOU are gay, and ask him if he knows anyone else who is.
That's a good one.
I was going to suggest perhaps inviting him to a "name that show tune" contest. If that doesn't work, ask him to name what brand of shoes the girl next to you is wearing. Failing that, you could just put on some house music and see if he can dance.
Ask him if he likes Bette Midler or Kathy Griffin.
Blow him a kiss
Ask what he thinks of a girl you both know but seriously most gay people know if someone other person is gay,That is what the gay people I know say.
Ask him his preference, women or men
see if he likes penises or vaginas you will definetly know then