How can you tell if your teenage son may be gay?

There is a lot of things about my son that I love and I want to understand him better. He is really into wearing women's clothes. But he has girlfriends. The other day one of his friends informed me that the hickey he had on his neck wasn't from his girlfriend but from a guy friend. How can I find out if this is true without messing things up and making him feel like he is on the defensive. He is only 17 and I am not sure how to handle this. Any suggestions for me?