Have you ever been at your after-school job flipping hamburgers and looked up to see the girl you like ordering at the counter for the third time in a week? If this has started happening a lot more than usual, and if she always makes a point to wave and smile at you when she visits, she might be hoping for a chance to talk to you. Then again, she could just enjoy a tasty burger, so keep your antennae up for any other possible meetings where you think she might seek you out. Maybe she's constantly walking by your locker to talk to you between classes, or you begin to see her regularly at one of your local hangouts. If you start seeing her places where you normally spend free time, she might be trying to get your attention.
Similarly, any girl who starts attending your after-school activities or sports functions to see you is sending a clear signal that she has feelings for you, especially if she's your loudest cheerleader. Maybe her afternoon run leads her through the neighborhood where you and your friends play basketball, and she just happens to stop and talk for a few minutes to see how you're doing. Taking note of your accomplishments, like recognizing you for winning a college scholarship, shows that she admires you and pays attention to what's going on in your life.
Bottom line: If casual meetings start happening more than usual, it's likely that she's got a crush on you, too.