A girl demonstrates her interest by the use of body language.
A glance, a touch, how she moves her hair out of her face, all these gestures indicate a level of interest that shows she is "open" to the object of her attraction. Flirts employ many of these devices as well; the difference is in the attitude in how it's done. Flirts like to have fun, and generally this is how they communicate on a similar level with other guys. Someone with legitimate interest in you will reserve these indicators of interest generally for you alone.
Flirts also use communication to participate in these little dances.
She will say something suggestive in a funny way and possibly tease her target relentlessly in a verbal duel meant as a challenge. This girl gets something out of attracting the attention of a man and will likely continue the game as long as she is getting something out of it. A girl who genuinely likes you may instead hint to something beyond just a playful tease. Her gestures may be more intimate, such as licking her lips or gazing at the man's mouth in invitation to a kiss.
The Road to Nowhere
As you become receptive to these unspoken signals, you quickly learn which girl is the flirt and which one has legitimate interest.
A flirt is amused by the interaction and generally has no plan to take it any further than that. She will eventually use her body language to "close off" from you. She will look away from a long, direct glance or change the subject if things get too personal. The fun is all in the chase. If her body language closes, such as her arms cross or she moves away from a touch, you know that it was just merely all in good fun. She will likely flit away like a social butterfly.
Because the dance of new-found interest is delicate, a girl who is really interested may in fact shy away from telling you in no uncertain terms how she feels.
She could become noticeably nervous even as she continues to demonstrate receptive, or "open," body language. This goes beyond the more self-assured behavior of a typical flirt. It also lasts longer. The relationship between a flirt and her target is fairly superficial and temporary, and will close the door on anything further than the moment. A girl who is truly interested will not brush you off after the initial flirty interaction has passed. She will want to get to know you and spend the time to do just that.