Even though reading a girl's mind is usually thought to be a superpower, if you can read her body language, it is just as useful, and this skill can be gained. We all know that there are gestures and facial expressions that we can control if we learn them, so reading her mind might be harder than looking for crossed arms or hiding her mouth.
Yet, there are expressions that are present for a fragment of a second, and the more one tries to hide her emotions, the more intense they become, so these are what you should focus on. Let me quickly describe a few basic emotions that you need to notice on her face to become a mind-reader.
If she is happy, then at least the corners of her lips will curl up with tight eye muscles and raised cheek. It might disappear in a second, but it can give her away.
However, her face does not only reveal secret positive feelings. She can also be negative towards you. If you see nothing in her eyes, but you see that only one corner of her lips is pulled up, then this might be one of the worst you can find. It tells that she feels contempt. This is something you can hardly change, so do not waste your time on her.
Yet, if you have only made an appalling comment or a sexist joke, you might see disgust on her face. It is usually temporary, and you can turn the situation back to pleasant with a compliment. Spot when she wrinkles her nose, and pulls one or both eyebrows down, and her lips are loose with one side of the upper lip pulled up. Then is the time to compliment.
See? It is not so much of a challenge to become a mind-reader.