Is he gay?

I think this guy I work with is gay, but I can't tell. He watches Rue Paul's Drag Race, and Project Runway, and has in depth conversations about both w/this girl in the office. He isn't very feminine, but has these slight mannerisms which are hard to describe. I think I caught him looking at my ass once. He seems to be very liberal and forward thinking and aware of gay culture. He never talks about girls, a girlfriend, or a significant other. All his roommates are girls. He looks at me with flirty eyes at times? Would a straight guy do that? How can you tell if they are actually flirty eyes, and based on what I've told you, is he gay? Under what circumstance would it be ok for me to ask if he's gay? Are there indirect questions I can ask?