I like this girl tried to get over it but can't so I just want to see how she feels before I ask her out and get heartbroken again.
how can I know
Forget the other answers, do it like I do, ask a friend, who knows a friend of her, to ask her casually if she likes you. Then tip both of them for their silence (1-5 dollars each, or buy them something for lunch)
Sorry mate, only one way to find out. Ask her how she feels.
You just have to ask her how she feels. But if you're shy you should probably get a friend to do it.