I wouldn't ask anyone outright if they were gay; I would just treat them like a regular person, and sooner or later, something will come up where you will know your answer.
I always say "wanna get coffee?"<br />
I can usually then guess by the tone when they answer, "like... As a date?" Lol. <br />
It helps if you are open about your own sexuality.
OK, if you are gay, just ask him. If you are straight...who cares?
Check the screen.
try, wow that girl looks so sexy ...and check his reactions quietly