Touch her and see how she responds. This doesn't mean grabbing her in a tight embrace, but rather just touching her shoulder or elbow or another, similarly innocuous part of her body. If she wants you to leave her alone, she will back away from your touch.
Look at her eyes to see if she's looking at you more often than at other things. If her eyes are flitting around the room, settling on anything but you, then she probably wants you to leave her alone.
Try to make plans with her. If it's difficult to slot yourself into her busy schedule, then she probably wants you to leave her alone. This is because her schedule isn't really busy. Rather, she just has other things to do (like watching her favorite TV show) that she ranks above hanging out with you.
Make a joke. It doesn't have to be that funny. Indeed, the lamer it is, the better, because if she's into you then she'll laugh at every joke you make. Conversely, if you make a funny joke and she doesn't laugh, then she probably wants you to leave her alone.
Ask yourself how easy it is to get in touch with her. Does she always answer her phone? Does she take days to respond to Facebook messages? If so, then she probably wants you to leave her alone.