People constantly send out signals to others using gestures and facial expressions. In fact, if you know how to look for it, nonverbal cues can sometimes give a more accurate look at how a person is feeling. Particularly, reading body language and other nonverbal gestures are one of the easiest ways to look for signs of attraction between individuals.
The best way to tell whether a person is attracted or interested is to watch the eyes. If a potential partner looks at you, then breaks contact more than once, she is interested in and slightly nervous around you. Also, pupil dilation indicates attraction and arousal. Frequent eye contact indicates interest as well.
Leaning forward or facing the body toward someone is also a sign of attraction. However, leaning away from another indicates discomfort or revulsion. Also if the shoulders point in your direction, it is a sign of interest.
Fidgeting is not always a bad thing depending upon how a person fidgets. If a person is straightening hair or patting down clothes, it's a sign of interest in the other person. Men, in particular, will consistently run their fingers through their hair. Women may twirl strands of hair on occasion.
Men will flex muscles, and women will sit up straighter with their shoulders back when they are interested in someone else. Better posture is a subconscious attempt at showing off the best physical features.