If she lets you touch her hair and doesn’t pull away she’s interested. I never let a guy touch my hair unless I like him, so if a guy can touch my hair I always let him kiss me.
Could be just me though, I dunno… =)
If she doesn’t pull away when you get close to her, and when you try to get close to her if she tries to get closer to you as well she likes you.
If she seems nervous around you and treats you differently; become happier when she sees you, talks more when you’re around, or if she avoids you (could be shy and doesn’t know how to act around you).
And like @nikipedia said, if she responds to your texts right away, she’s interested.