This is never a good situation. Guys always lust over the girls that they don't have, and trust me most of the time it is all sexual. They will sweet talk you, have sex, and then stop talking to you because they would never leave their "happy home" over you. You will never have the advantage or the upper hand, you are just a girl they screwed. Do not involve yourself in a situation, you do not need to be in.
if any guy makes eye contact these days its usually a dead give away that he likes you, but to be sure, see how he is around his other friends. if it seems as if he makes more eye contact with you than his other friends then yes, in my opinion he likes you.
Nothing. As long as he has a girlfriend, I suggest you don't act on your feelings. I feel it's disrespecting towards their relationship and his girlfriend. If he makes a move, how hard it is to resist, I suggest you try. Put yourself in her position. If you had a boyfriend and a girl had feelings for him, would you appreciate it if she acted on them?