How can I tell if she likes me more than a friend?

Question

There is this girl that I like and I can sense she likes me because we talk on instant messenger alot, we text one another alot. If we dont meet up she gets upset that she didnt get a chance to see me. Recently I asked her to be my date to my formal that I have, she said yes. Me and her decided like a long time ago that we didnt want a relationship with anyone at all at that time. And now I feel that things are changing that I have strong feelings about her bc I like her more than a friend. Whenever something is wrong or if she needed help she calls me. Does she like me a lot? Would she be my girlfriend? How can i find out, what can i do or say? Thanks