Start casually and then build up to asking her out properly. Start off by complimenting her. For example; You could say I think you're really hot/ pretty, or get more specific, like, I've never seen eyes that blue!. If she seems to like being complimented, then you can move onto the next stage.. See if she makes physical contact with you. If she hugs you or seems to be touching you a lot, then this means that she is really into you. For example; When you sit next to her she leans into you, or she lightly brushes your arm/ back when speaking to you.