How can I tell if people have a crush on me, and what do i do?

Well, for starters, you may have answered your second question: telling people who have feelings for you that you're not in a place or looking for a relationship is often a good, honest, delicate way to turn them down. In my experience, it's also generally a good idea to add how much you appreciate their friendship and invite them to keep interacting with you as a good friend. (If you want them to, anyway.) Be clear and firm when saying you aren't interested, but offer a sort of olive branch, if it feels right.

As far as reading others' signals, if you're not interested, I wouldn't address the situation unless it makes you uncomfortable; give them a chance to either take your hint (and thus stop flirting) or tell you directly about their feelings.

If you find someone's advances make you uncomfortable, though, absolutely find a way to make it clear you want them to stop, whether that's just saying so or telling one of their friends or anyone else who can help.