If you're trying to figure out if your friend is gay because you want to date him (as a woman), it's important to remember that just because he might be interested in men doesn't mean he's not interested in women. For example, his family may be violently homophobic and by deciding that he's gay, you may out him by accident by treating him different or implying something without meaning to.

By 'outing' them, even to yourself, you may be putting them in danger. You may want to know if your friend is gay, but there may be a good reason why they're staying closeted. People have good reasons to stay closeted sometimes.