How do I know she means it?

Time will tell. Girls like the idea of being in love, so you can't know for sure. On the other hand, I think emotions are stronger when you're younger, so she may well have very strong feelings for you.

Don't say it back to her unless you mean it because that's mean!! Ask her did she truly mean it or just thinks she is? And what did she say it like?? Like I do be having a laugh with guy friends and they would do something funny and ill be like awhile I love u! and same they would say it to me if I said something silly or stupid! or did she say it real serious and meaningful were you were sure she meant it?

Well you don't know exactly if she means what she always says. You could discuss it w/ her, if you'd like. Saying " I love you" means a lot to people and if you feel the same way about her you should let her now. Having a guy tell you that they love you is something every girl likes to experience in her life...especially if she feels the same way in return. I hope everything works out for you.