How do you know if a girl flirts with a guy to be nice or because she likes him?

If she likes you, when you look at her ( if you like her ) and she is looking at you and she quickly looks away. Or laughs at every joke even if it is lame. She will have foot fights or paper fights with you. If she only wants to be friends she won't look at you very often, and won't laugh at your lame jokes.