Mistaking whether a girl is flirting with you or just being friendly can lead to serious disappointment, not to mention embarrassment. Know the signs of each before you make a move in the wrong direction.
Notice how exactly she flirts with you. There's a difference between friendly flirtation and romantic flirtation. If she's smiling, laughing at your jokes, or looking for a place next to you at the lunch table, then she's interested in you, but there's no guarantee she wants you romantically. However, if her gaze lingers, she goes out of her way to find a chance to talk to you, or mentions mutual interests she wants to enjoy with you, then these are more reliable signs that she's genuinely romantically interested in you.
Know what her motivation is with you when she's giving you attention. If you have something she wants, are in a position of professional power, or you have a social status that requires her to be friendly -- then she may not be interested in you romantically. However, if you can't think of a reason she would feel obligated to befriend you, she may be flirting.
Don't confuse kindness or sympathy for romantic interest. Consider the way you have presented yourself to her. If you have a sob story and she knows about your personal struggles, or if kindness is one of her shining traits -- then she may be just friendly and not romantically interested.
Ask your friends, her friends, or anyone who has known her for a while if she is this friendly with everyone. They'll be able to give you some context in which to put your own interaction with her.
Watching the woman as she interacts with other men can give you the answer you're looking for. If she's just as flirtatious with other men as she is with you, or her friendliness toward other guys makes you feel jealous, this is a red flag that she is just being friendly with you or just flirts with everyone. However, if that sparkling smile and lingering gaze is something she shares only with you, chances are she's really flirting.