If a girl's interested in you but isn't sure you're interested in her, it can be less clear cut and she might seem to contradict herself. But over all...
1. She makes an effort to spend time with you. Alone.
She's interested in what you have to say and takes the initiative to ask you questions about yourself, about what you're interested in, etc. She likes how you act in groups but she would rather find out who you really are.
2. Body language: brief invasions of the personal "bubble", touching you on the arm, messing with your hair, etc. When you sit together, do you sit close enough so that you could touch? Knees? Shoulders? And does she move away when you're this close? Because "just friends" usually try to maintain a "no misunderstanding" distance. If not, that's a good sign, unless you're trying to fit 10 people in a car or she's super laidback about personal space.
3. Reading between the lines
Watch for her reaction when you say something that COULD somehow imply that you're interested too. Abnormal reactions include going completely still, suddenly taking interest in some object, suddenly playing with her hair or fixing her clothes, etc.
Everyone else has good answers too that could help a lot. Getting a second or third opinion really helps too. I just tried to give you the "shy girl" clues. =D