Ask them.
How do you know if someone likes you?
They'll flirt with you.
Asking them is obviously the best way out, but if you are not confident enough to as the person, then body language can help you get the answer. It is the non-verbal communications that can help you get the answer to your question. You can look for a few cues like feet pointed to you while talking, gazing into your eyes, and many more...
You could flirt with them. If a girl responds to your flirt, she likes you. If not, sorry, not into you.
They ask you on a date or tell you they like you.