Here's a surefire way for any person to tell if they are gay or not. Get them to ask themselves the following question:
Q: Do I fancy people of my own gender?
If they answer "Yes" then they are. That may sound a little facetious, but there are no other guaranteed ways of telling; for instance, I know a bloke so camp he's signposted for tourists, but he's straight. Many people don't believe him, but that doesn't change his sexuality! One of my other mates is not in the slightest bit camp but *is* gay.