Answer:
Firstly, it's pretty much impossible to tell if someone is a homosexual without actually asking them. Dressing in the clothing of the opposite sex might just mean that they like dressing like the opposite sex. There are such people as transvestites, which has nothing to do with sexual orientation, but all to do with dressing like the opposite sex.
Secondly, the only thing all homosexuals have in common is that they fall in love with people of the same sex.
The point is, that you will not know until they choose to tell you. You can suspect, yes, but there is no sure way of saying "he or she is gay", unless you see them being intimate with their significant other. Even so, they might be bisexual (attracted to both sexes).
If your question is concerning yourself, my first advice is "don't worry". A lot of (straight) people go through periods of being uncertain about their sexuality (a lot more than would actually admit it).
If you find yourself strongly attracted to people of the same sex, yes, you might be homosexual. A sudden urge to wear pink (if you are a boy) or cut your hair really short (if you are a girl) doesn't really mean anything.