People are usually very aware of their facial expressions, but they don't necessarily realize that their bodies are sending pretty loud messages, too. She may be giving you hints that she's not interested in the way she positions her body.  If she turns her body away from you, or leans back or away from you, it's a sign she may be uncomfortable.  She keeps a barrier between you. If she always positions herself behind a chair or a table or holds her purse or backpack in front of her like a shield, she might not be interested in you.  If she walks away when she sees you or appears to be avoiding you, she is not interested and you should not try to follow. Making eye contact can actually be quite intimate, and some girls may look down or away because they are shy. But if you find that her eyes are always darting around when you talk to her, as though she's looking for her friends, she not really feeling the conversation and you should politely excuse yourself. Remember that one conversation during which she seems distracted or isn't looking at you doesn't mean she doesn't like you. But if it seems to happen every single time you try to talk to her, something is up. She's flirty and talkative during class or when you have a shift together at work. Then you run into her when she's out with her friends, and she looks right through you. If there's a huge gap between how she treats you in private and in front of her friends, she might just be flirting with you out of convenience.  She might also be too self-conscious or shy to say hello to you in front of all of her friends. Are her friends are constantly glancing at you, even if she's not? It might mean she's talked about you to her friends. Try making the first move and see how she reacts. Say hello when you see her out in public. If she seems embarrassed, is rude, or flat out ignores you, then you can be pretty sure she's not interested. Even if it's not that funny, a girl will usually laugh when a guy she likes makes a joke. Keep in mind, this is something most friends do, too, so it doesn't automatically mean she's into you. But if she doesn't even giggle at your funniest stories, you might need to find someone else who appreciates your humor. If she doesn't crack a smile at your joke, it could be because she's having a bad day (or it was a terrible joke). But if your jokes are met with silence day after day, then it's time to move on. If you ask her to the movies and she shows up with three friends, it might be a miscommunication. But if it happens every time you invite her to do something, it might be her way of letting you know she just wants to be your friend. Keep in mind some girls are not allowed to date until a certain age, and she may need her friends to tag along so her parents will let her hang out with you. But she should definitely let you know ahead of time that this won't be a solo hang.
Try to read her body language when you're together. Check if she avoids making eye contact with you. See if she treats you different when it's just the two of you versus when you're in public. Watch her reaction when you tell a funny story. Notice if your dates always turn into group hangouts.