Very simple rule. This will help you out. If you've spent some time talking to a girl, you will always remember the feelings after you walk away from the situation. Those are the feelings that linger in girls, too.

If you feel awesome and walking on air, the person you were talking to was interested. If you're feeling like this but on your own you're on MDMA.

If you feel confused or weird after leaving, the chances of anything happening in future are squat.