If a girl likes you she will smile a lot near you and be happy when your together. She will laugh at your jokes unless their really lame. She will try and be near you often. She will flirt.... (not all girls but theres some clues)
You will know that a girl likes you when she laughs a lot at your jokes, she will also give you random hugs and touch you often. Most girls will also keep looking at you and will talk about you a lot with their friends.