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.
I am beginning to suspect that it is surprisingly common for intelligent, competent adults to somehow make it through the world for a few decades while missing some ordinary skill, like mailing a physical letter, folding a fitted sheet, depositing a check, or reading a bus schedule. Since these tasks are often presented atomically - or, worse, embedded implicitly into other instructions - and it is often possible to get around the need for them, this ignorance is not self-correcting. One can Google "how to deposit a check" and similar phrases, but the sorts of instructions that crop up are often misleading, rely on entangled and potentially similarly-deficient knowledge to be understandable, or are not so much instructions as they are tips and tricks and warnings for people who already know the basic procedure. Asking other people is more effective because they can respond to requests for clarification (and physically pointing at stuff is useful too), but embarrassing, since lacking these skills as an adult is stigmatized. (They are rarely even considered skills by people who have had them for a while.)
This seems like a bad situation. And - if I am correct and gaps like these are common - then it is something of a collective action problem to handle gap-filling without undue social drama. Supposedly, we're good at collective action problems, us rationalists, right? So I propose a thread for the purpose here, with the stipulation that all replies to gap announcements are to be constructive attempts at conveying the relevant procedural knowledge. No asking "how did you manage to be X years old without knowing that?" - if the gap-haver wishes to volunteer the information, that is fine, but asking is to be considered poor form.
(And yes, I have one. It's this: how in the world do people go about the supposedly atomic action of investing in the stock market? Here I am, sitting at my computer, and suppose I want a share of Apple - there isn't a button that says "Buy Our Stock" on their website. There goes my one idea. Where do I go and what do I do there?)