yes, i think a gay guy could pretend to like girls but i don't think he would pursue girls if he wasn't seriously into them. It could be that this guy is just uncomfortable around the opposite sex and doesn't quite know how to act. If he is coming up to you and trying to be around you, i would guess he's straight.
Ah, as I recall the term for a girl who dates a gay guy is a "beard." So yes, gay men and women and bi men and women and pan men and women and all the other multitudes of sexualities can pretend to be straight. Dating heterosexually is a good way to stay in the closet for them since it seems like solid evidence that they are not of another sexuality.
How to find out if a guy is gay, there's no telling. You might just want to ask around or even braver, ask him face to face. I wouldn't flatout assume that a guy was gay if he seemed closer to his friends. A lot of boys are just like that.
Yes of course - men/women do it all the time. Some times they are nervous/scared to come out. Sometimes they are still struggling with the am I? Am I not?
Best bet is to ask him. Communication is key =)
And if he is gay that doesn't mean he isnt awesome, so he'll still be a great friend.