How will I know that she knows?

You won't know for sure. It might not necessarily be obvious. Only that if she's not interested - she'll feel uncomfortable around you and might keep her distance. She might not confront you either. I know I wouldn't! I would just keep away from him for a while.

It depends on whether or not she likes you back, and of course her age. Assuming you're young, if she likes you, she will start flirting with you, however, it might be very discrete, if she's shy. If she doesn't like you back, again, depending on the kind of person she is, she might feel a bit awkward around you, she might even start avoiding you, or she will like the attention and provoke our liking her by flirting, but making it clear that she doesn't fancy you back.