How do I tell when a girl wants to be more than friends?

I have had a number of close female friends where I have been unsure of their interests or intentions. I'm not looking for anything more than friends, but sometimes I get hints that they might be interested, or..I am reading too much into things. For example, there is friend I have been hanging out with pretty often recently I haven't been sure of. She has a boyfriend she seems to like pretty well and often tells me about, but she also does things like push her breasts up on my arm while we were looking at a map together yesterday, and she always gives me a hug when saying goodbye. I think I've indicated I just want to be friends, but maybe I need to clarify?

TL;DR: How can I tell the difference between when a female friend is just being comfortable/friendly, and when she wants to get with me?

edit: Maybe this wasn't clear, I just want to be friends with her.