The only way to know for sure is to ask!

Don't just blurt out the question, though. Flirt with them and no one else for a while. (Not just a few hours, a few days or even weeks.) This will make them think that you're interested in them. Once you feel comfortable, go somewhere quiet with them, and talk how you are feeling about them. Be openminded, and don't put your whole self worth into what they say. It may feel awkward around them for a bit if they don't feel the same way, but just act like nothing ever happened. They will probably do the same.