Look at your music, compare your DVD or lesbian themed material to a straight friend of yours? This is a good sign that you might be attracted to the lesbian community.
If you have dated or do date boys do you find yourself making up a lot of excuses as to why you shouldn't be dating them. Are you overly picky or extremely shy if a boy hits on you? Does it make you uncomfortable if a boy hits on you? These all could be signs that you're simply not interested in boys in that way.