It's flattering when a woman has a crush on you, though if you don't reciprocate her feelings (or are in a committed relationship with someone else), it can also be quite awkward. Before you can act on a woman's crush--either by pursuing the relationship or letting her down as gently as possible--you need to spot the telltale signs.
A woman with a crush will exhibit certain undeniable physical signs when she is attracted to you. She'll often cross and uncross her legs, or twirl her hair around her fingers. Her shoulders will lift up and she'll turn her toes inward. (That last sign may also mean she's a little scared of you, so be gentle if you spot it.) She'll reveal her hands palm-sides up--a sign of vulnerability-- and may move closer to you as you speak. Like men, women also tend to bow their forehead when they're interested in you. And of course, she'll smile at you a great deal, inviting you to come closer.
People with crushes tend to flirt a lot, but there are more specific signs when a woman speaks to you that let you know she's interested. She'll make unbroken eye contact and maintain it throughout the conversation; since she's interested in you, she wants you to know she's listening. You may notice that her eyes dilate when she looks at you, with the pupils growing bigger (it's an involuntary reaction which has nothing to do with the surrounding light). She'll hang on your every word, no matter how dull or trivial the conversation, and make efforts to entertain you and laugh at your comments. If she's nervous or doesn't seem to know what to say, this may indicate a crush as well, especially if accompanied by the physical signs.
If a woman has a crush on you, you may find yourself bumping into her at unexpected times. Those "chance meetings" may be an effort to be around you more, and while it may grow exasperating if it happens too often, it stems from genuine attraction. The meetings will also be tinged with reasons to get together with you--for instance, asking you if you know a good restaurant in the area, and then inquiring if you're hungry. She may take the opportunity to do nice things for you during such meetings, like buying you coffee or paying for a meal you have together.