How do you know when the guy that you're dating is bi?

I've been dating this guy for three weeks and he's so sweet and charming. We have great dates, but in the bask of my mind, I always seem to think that he is also into men. He's told me about having a few gay friend, which I'm fine with, but I can't help but to think that he's also be intimate with them. Has anyone else had this experience?