I'm sure sycld can help you out here. I'm bi myself, though, so I'll give it a shot.

Personally, I can tell a gay guy on sight but, since that appears to be an issue for you, my suggestion would be to head to a gay bar or club. Most straight 'allies' in gay bars are usually female so you can (usually) be sure of a 98% homosexuality rate :P. Besides, if you hit on someone there, it's their own fault.

Otherwise, just strike up a conversation with a guy you're interested in and ask? I can't see too many people being THAT homophobic these days.