Sorry to disappoint you mate- but it's impossible! Girls are the most unpredictable of all God's creations (trust me- I'm a girl). But, if you really want to know a girl inside out, then the best way is to become her best friend. This works especially if you're a boy, because girls tend to tell boys things they might not always tell other girls. So, listen to her, and talk to her, and get to know her, and you'll start to get a rough idea of what's on her mind. Also, pay attention to her body language, and expressions. It works sometimes! But be warned, no two girls are alike, and sometimes, even the girl doesn't really know how she feels, and when she does, she probably won't tell you... It's an art that takes time and practice... You could just ask her what's on her mind, I suppose!