TL;DR: ASSUME ATTRACTION.

This should be pretty standard knowledge, but so many questions have come up lately along the lines of "does this girl like me??" or "how to tell if that one girl at uni that i saw on the sidewalk likes me" or "girl said hello, does she like me?" that it apparently needs to be reviewed.

I see it on r/askseddit constantly. The quickest way to show that you are needy and insecure is to start a post by saying, "i'm a pretty confident guy", and then ask us how to tell if a girl in some particular situation likes you or not. A truly confident guy knows better. If you haven't specifically asked her out yet, then you definitely aren't assuming attraction. If you are assuming attraction, why haven't you asked her out yet? Confident men don't sit around wondering; they take action.

Guys, this is so easy. If you are interested in her, ask her out. If she says no, move on, but you'll never get a yes without asking.

Assuming attraction should drive your action. You could waste a bunch of time trying to figure out if she likes you, or you could be the guy with the balls to find out while the other guys in the room sit around wondering. Assume attraction and act on it.