Most girls have different tells for flirting. They can play with their hair, hit you playfully, tease you, laugh or giggle a lot. Some girls even get nervous when they're trying to flirt. If you know the girl, then it'll be pretty obvious because she will most definitely be acting different. But, it's different with most girls. Every girl has her methods.

With the help of some tricks and technique you can know easily girl is flirt or not. Recently I found out a cool book called “System”. I've read so far on picking up girls. I got it on amazon. The author is Roy Valentine. I actually used the stuff from the book to pick up a girl the same week I read the book. Highly recommended. Hope this help you.
Good luck!