I think I've got pretty decent gaydar(tm). I usually base my assumptions on:
1. body language
2. voice and speech cadence
3. type of dress
4. who this person hangs out with--- mostly groups girls all the time, or guys all the time?
Then of course as Chris mentioned some people are just completely un-guessable. I'm 99% sure one of my friends is gay but it's taken me eight years to determine this. I'm guessing based on the fact that he has NEVER said anything to me about any girl, EVER, as well as his involvement with local gay groups. And I just get that feeling from him
Some people are tough to read!