The only way to find out someone's sexuality is to ask. It shouldn't be awkward, but in fact is normally a way to show someone respect, more so than any other technique ( i.e. poking around in his past or asking other people). If you're worried about it, start with something empethetic like, "lately I've been wondering about sexualities, I've been wondering about how someone knows if they're straight or not, have you exerienced anything like that? Do you identify as straight?"

If I were you I would not confront him with a direct question about his sexuality. I would just continue with him for a while and observe what is happening between him and other men. If it spoils your relationship with him, that would be the time to back away from him as being an unsuitable partner. Labelling people as bisexual can be very misleading: they are often not attracted equally to both sexes; also they can change

Barry I don't agree with your advice at all. Part of being with a person is being open and honest. You don't have to label someone to make an inquiry. I asked my husband if he was straight after a few weeks of knowing him. Sexuality shouldn't be this taboo judgemental thing. If you are close with someone I feel like it is appropriate to ask. Also watching your boyfriend for signs of gayness is really kind of creepy. I would be offended if my partner was watching me and judging my interactions instead of just posing a simple question. If you are dating someone you have the right to ask personal questions. If he was a random coworker or associate I could see being uncomfortable about out of the blue trying to out someone. Especially because it's noones right to go around labeling and outing people. But asking your partner their sexuality is an appropriate question, especially as if they aren't completely hetero it is probably a large part of their identity.