I would like to know how to tell if a girl likes you

No one way, if she talks to you, does things with you in group.
At different ages, it works different... what is your age.

You will know the girl likes you if she does everything a bit extra for you then others, for instance- extra care, extra talks, extra attention, extra mile walk for you, extra extra... etc. (if she is doing only one or two things extra don't consider it then, if she likes you she'll make you feel that you matter for her.)
I know every girl is different but I know few girls who does extra things for the one they like most.