How to ask a guy if he's gay...?

So I'm bisexual, and out to everyone I know. The guy I like (Henceforth: Don) evidently didn't know until recently, when my best friend (henseforth: Mark) told him. (I told Mark that he could tell anyone. Like I said I'm out to everyone I know, but some people just haven't got the memo yet I guess.) Now, I think Don might be bi (if not gay). He just gives me that vibe, you know? Mark agrees with me that there is a chance Don is gay. However, now that Don knows I like dudes, I don't know how to ask him if he does without being creepy, or too "I want to fuck"-ish.

If it is at all important, I am a Junior and Don is a Senior in high school. We only have one class together, but we talk quite a bit in there, and sometimes we'll talk in the halls. He is also like a 9 and I'm like a 6-7...but I've been told I have a great personality.

Any advice would be greatly appreciated!