if she is your girl friend then she really likes you. check if she doesn't like to eat around you, if she doesn't then she really really likes you. if you are not dating her then she prob likes you but not to be stalkish but see if she is really talkative around ugly guys or her Friend's.
If you are dating or hanging-out together then it could mean she likes you, but it's not enough to say "yes this girl likes you a lot because her head is on your shoulder." If you aren't dating then ask her out on a date and if you are dating then you know she already likes you a lot!
I also liked a girl who was more popular than me and now she is my girlfriend. If you are really serious about dating this girl then you should just be yourself and if she likes you for who you are then go for it and ask her out!