How do you know if a girl is friends with a boy or in a relationship with him?

Honestly, it is almost impossible to tell. It really depends on the girl.

If the girl is the popular type who gives lots of hugs:

-she is most likely just being her flirty-self, does that often

Although if the girl isn't really a hugger, and you see her hug someone, she might like him.

For any girls, if he is all she talks about, then yes, she likes him and is overly obsessed.