A lot of people say that bodies can't lie. Nature has programmed humans with a complex set of non-verbal flirting signals that just flow freely when people are interested in someone. These range from the widening of the iris when looking at the object of desire to more overt displays such as smiling or touching.
Here are a few body-language cues to watch out for:
She points in your direction with her leg, foot or shoulders.
She leans toward you while talking.
She plays with or tosses her hair.
She fidgets with a piece of jewelry (like an earring) or strokes the stem of her glass.
She keeps her eyes locked on you while she talks or drinks.
She mirrors your body movements (for example, if you put your hand on the table, she quickly does the same).
She smiling when you check her out.
If you become aware of a cluster of these signals, you can almost be sure that she's giving you the green light for romance.