How do you tell if a girl is flirting with you online?

that's a good thing! You should put a really romantic song on her facebook and see how it goes. One that's something along the lines of "You Belong With Me" by Taylor Swift, but you know, not one by Taylor Swift. Have it be a not so popular song so she thinks of the lyrics as something you're saying more than just a song she hears on the radio. If she takes it badly though, maybe it's just a song you happen to like? ya know...