So you want to know the truth behind the eternal question, "How do I KNOW she likes me?" eh?
The answer is you never will. Girls do not make sense. Girls do not think logically and do not do things that 'normal' people would do. Girls are not normal and should not be treated as normal.
You could meet a girl tomorrow, end up marrying her, grow old together, die, and be buried together. Does she really like you though? Most would say yes, but hey, what if she was only with you because you were rich? You will never really know.
Here's what you're going to do for me, depending on how well you know each other. Obviously you have talked before if you are even asking this question.
You will walk up and initiate a hug while greeting.
You will NOT start the conversation with "How are you?"
You will tell her to put her number in your phone because you two are going to do something crazy, insane, and completely illegal this [insert day].
Call her [day] and tell her what you two are doing. If she has plans, strike one. It's the same as baseball, and don't give her mulligans.
Hang out, isolate, make a fvcking move, get rejected? Oh well, move on. It worked? Great. Continue.