Although a woman's smile should not be taken as a clear sign that she likes you, a seemingly uncontrolled or constant smile should be taken as a signal of delight.
The eyes are a window to the soul, and if she's checking you out it's a good sign. Not only will she be watching your every move, either from afar or while talking, she will be scoping out your eyes and lips on a consistent basis. Another form of contact comes from her need to touch you. If she is touching your hand, arm, leg, face or even the tabletop right beside you, she is probably flirting with you.
If she is laughing at all your jokes, even when they're not funny, she is probably tickled with your behavior and expressing her flirtatious interest. Playing along with your jokes, plus the use of coy sarcasm, can also indicate flirting. Use your own judgment as to whether she is interested or just being playful.
Women want to be noticed, and they try subtle and non-subtle ways to achieve this. Something as simple as twirling her hair with her fingers, or excessive playing with her hair, are subtle hints to get you to pay attention to her. While talking, she might fiddle with her glass, her hair, her accessories, her clothes, or even the clothes of the person she is talking to. This is an absent-minded way to keep the mind always stimulated and alert. Another subconscious flirtation comes if she makes an effort to straighten her blouse or skirt, fix her hair, reapply makeup or make any other gesture of physical tidiness before approaching you. This means she wants to look good.