How You Can Tell If A Girl Is Flirting With You

Short answer: 
Five key signs of female interest: Eye contact, smiling, touching during conversation, laughing at your jokes, sincere questions about you. If she isn't giving off 3 or 4 of these signals, she isn't flirting.

Long Answer
Reading signals takes practice. Try this exercise. Walk around the campus or your neighborhood and look for young men and women interacting. Look for situations where the girl seems completely absorbed by the guy and kind of blind to the rest of the world. What kind of feeling do you get from her? Do you get the feeling, "She is really interested in that guy?" Watch more interactions. Each time pay attention to the feeling invoked inside you. You will see some where the feeling you pick up is one like: "she is bored", "she is disinterested" or "she is annoyed."

When you begin, your success rate at reading others may be poor, but it improves quickly. You just need to tolerate your early failures, learn from them and keep trying. As your confidence in your people reading skill goes up, you will feel more comfortable acting when some one shows interest signs in you.