It's not always this clear-cut, but if a guy touches you at all while he's talking (brushes your hand, etc.), makes an unusual amount of eye contact, or makes a point of being alone with you, it's flirting. If he's talking or joking about sex, it's more likely to be flirting.

How do you become the kind of friends who see each other outside of class? That used to confuse me SO MUCH. The easiest way to transition from "person I've spoken to" to "actual friend" is to say "You want to get lunch together sometime?" It's also possible to ask "are you going to event X?" (I used to find this step nervewracking. But remember, most people are not offended by offers of companionship. Most people want to make new friends.)

Also, notice how people hang around after an event. Most people don't leave right away, briskly. They sort of mosey and talk. If you're like me, your instinct will be to think, "Well, I'm done with that, time to go do something else." But more social people spend a colossal amount of time just hanging around, and they exchange more closeness that way. You can't make friends with people who only see you in brief bursts.