There is no reason for you to know. If it doesn't bother you, then let him come to you in his own time, and if it does, then you need to get over that and let him live his own life.
I only came out to my parents because I wanted to share my life with them, and they had made it clear that homosexuality didn't bother them.
If you are worried about him having sex, there isn't much you can do about that even if you did know he was gay. I remember reading somewhere that not only is the average age that people lose there virginity is 16, but 47% of teenagers will lose it at home, in the middle of the day. I certainly lost mine while my parents were just in the other room, and they knew I had my boyfriend in there with me with a open doors rule.
Making rules for him will just upset him, and drive him to make unsafe decisions to get what he wants.
If you are really just worried that he may be doing something unsafe, then maybe a general safe-sex talk would be best.