Perhaps the most obvious sign could be that he is reluctant or uncomfortable with intimacy with a girl. There may also be some clues in the things that he says. If you don't know for sure if your boyfriend is gay or not, it might mean that he is 'not out' yet. Be patient and sympathetic. If your relationship is good enough and you feel there are strong grounds for believing he is gay, try and bring the subject up and see what reaction you get. Obviously be careful, as it might not be the reaction you were expecting.