The way I know a girl is into me is simply by looking at her. No, not staring at her, not glancing at her.
Looking at Her!
More specifically through your peripheral vision (focusing on the side of your eyes to look at a girl).
If your around her and shes completely checking you out, she either likes you or is interested. If shes kinda glancing at you, then she recognizes
YOU! Take that as a chance to go enchant her world by getting to know her, not by showboating and bragging.
If shes acts super sexy around you
ie. is walking like a Victoria Secret model past you, is looking uber fine, is talking to other guys around you (preselection), then she is either looking for a boyfriend or is specifically targeting
YOU!
I tend not to go off how shes acts
ie. shy, confident, aloof, excited, because any of those could reflect how she feels today or she can be anyone of those and still not be interested in you or completetly interested in you (girls can have game of their own). You can't even go off how she changes when she meets you
ie. was excited but now is shy, was aloof but now is excited, all this says is your making an impact, but douns't mean she likes you ( can mean your doing something right or wrong, she was excited but is now aloof, shes probably bored).
Another more direct way to do this is to simply go up to her and state your interst!
An example:
You

stopping her, hand on shoulder or stomach) "Hey, do you mind if you walk me to clase?" (sexy eye contact and smile)
Her: "Umm... no, i don't know you (laugh)!" or "Sure!" *either ones fine but one shows confusion while the other shows enthusiasm*
You: Relax, I was merely playing. (pause, stare deeply into her eyes, still smiling) Actually, I thought you were cute and wanted to say hello. I'm *Insert Name*.
What this does is it
breaks the ice, gets her to laugh (positive emotion, tension breaker), gets her intrigued, lets you know if shes interested and vice versa, breaks touch barrier, and opens for you to easily move her (investment). If she goes along the lines of "...ok" then shes not interested or its never happened to her before. I haven't had this happen before, but I realize it could, so its something you have to learn to respond too.
In all, if you think she likes you, then go find out!
With best intentions,
Luke