The others said it all real well, not much I can add but body language is a clue IF you have enough social experience to pick up on subtle clues which at your age you probably don't have yet.
Some possible signs might be of some clue- the fellow doesn't talk about girls or dates, doesn't have a girlie calendar on the wall, he may like what you could consider feminine things.

The real safe way to "ask" without asking would be to start a conversation with him in private and then maybe work-in something like bringing up a recent news story or letter-to-the -editor like;

"Hey, I read this story the other day in the paper about that gay marriage thing, isn't THAT something! I don't really care what people do in private or who the marry you know?"

If he says something like "Oh man those F****'s are just CRUD and should be shot!

Then you get out of the conversation real graceful!

But if he says something like;

"Yeah that's real cool, I think gays SHOULD marry, heck my brother is gay"

Then you have a shot at working the conversation