This question is not clear. If you just like her and happen to know that she is casting a net on a large scale, you can tell her directly and then make a decision by watching her reaction. If you like the other person, but she is also being nice to other boys besides you, then you should have a good talk with her and see what she thinks. See if your conversation can stop her from doing this. You can continue dating if you can. Otherwise, you have to seriously consider whether to continue this relationship.