Well... I feel horrible, but I'm kind of pro at this. If you're talking about a girl online, it's really super hard to tell because you've got the computer between you. In person, there's a lot that can give it away.
When someone's trying to flirt with me and I'm just not that interested in them (oh my gosh, I'm a terrible person...) I usually don't try to move the conversation along. If I really like somebody, or if a girl really likes you and wants to be with you, she would most likely be really eager to try and keep you talking. It's really important to use your gut though, because if I'm totally into a guy, sometimes I have a really hard time coming up with interesting things to say. If you feel like she's kind of nervous, that most likely means that she is and that she's kind of in to you.
If she's joking, that's just plain good. That means that she's trying to be adorable and make you laugh - ie: she likes you.
Girls can play with their hair in two distinct ways, and I know this because it's an extremely measured procedure. If she's examining the ends of her hair, like she checking for split ends or something, that means she's soo so bored. If she kind of runs her fingers through her hair and flips it, it usually means that she's trying to do a sexy-thing, and that's good.
Some girls will touch your arm, and that's good.
If she keeps looking around desperately when you're trying to talk to her, like she wants to find somebody to rescue her - that's bad.
If you get the feeling that she's laughing at you instead of with you when you make a joke, she usually is
Oooh, I wish I could help you more, but my main thing is just to follow your instincts. If it feels like she likes you, then she most likely does. If it feels like she doesn't want to be anywhere near you, or she's always irritated with you, then sadly - she is.
Message me if you have any really specific questions, and good luck!!