A girl who has a genuine interest in you will find ways to do nice things for you. If she sees that you're down about a grade, she'll offer to help you study for the next test. If you both share an interest in a band, she might buy a pair of tickets to a concert and offer one of them to you. A girl who's into you will show concern or act protective of you. If she sees that your current love interest isn't giving you the time of day, she'll swoop in to comfort you -- maybe hoping that you'll notice her, instead.
Another obvious sign that a girl likes you is if she remembers you on significant holidays or your birthday. If your school sells candy cane-grams at Christmas and she buys one for you, or if she brings you a red velvet cupcake from your favorite local bakery for your birthday, this shows that you're on her mind. Some girls might choose to give you a gift that sends an obvious message, like a handmade, heart-shaped card for Valentine's Day. Others might give you something a little more cryptic, like a mixed CD full of your favorite songs. A good rule of thumb with gifting is the more thoughtful the gift, the more likely a girl is interested in dating you.