Your mother probably taught you that it's rude to stare. But when you negotiate a business deal, close observation of your opponent makes sense.

By inspecting your opponent's every physical move, you can often determine whether he or she is holding something back or not telling the truth. The key is not to stare so much that you make your opponent uncomfortable, but to be aware of his or her movements through casual glances and friendly eye contact.

It will almost certainly give you an edge.

What should you look for? Experts who study body language suggest a two-step process. First, identify a subject's mannerisms during the initial, friendly stages of a discussion. As the negotiation unfolds, see whether your opponent suddenly adopts different behavior. "You have to watch people a long time to establish what their baseline mode is," said David Hayano, author of "Poker Faces." "Once you know how they normally behave, you may be able to tell when they start to put on an act."

Hayano is a retired professor of anthropology at California State University at Northridge, who has analyzed the body language of poker players, and he's found that the rapport-building stage is a valuable time to study your opponent. Why?

Because that's when you get to know someone's "natural" behavior. "If you are dealing with a very talkative executive who all of a sudden gets meek during the heat of the negotiation, then something strange is going on," he said. It may be a clue that your opponent is hiding something; other clues are exaggerated movements or excessive enthusiasm. Hayano says that in poker, for example, a player who throws chips forcefully on the table or suddenly behaves in a brash, aggressive way may be masking his being stuck with a weak hand of cards. The same goes for executives who loudly and repeatedly proclaim that they're making a major concession, when in fact they're not giving up much. "When you're negotiating with someone who starts overtalking and backslapping, this can mean they really have little to offer," Hayano said.

A range of nonverbal clues may serve as red flags during a negotiation. Experts suggest paying special attention to a person's hands and face. "There are many revealing body signals that may indicate a hidden agenda," said Donald Moine, an organizational psychologist at the Association for Human Achievement in Rolling Hills Estates, Calif. Examples include hair pulling, lip biting, eye blinking, gulping and throat clearing. According to Moine, a negotiator who starts breathing rapidly may not be telling the truth. "The way to tell how someone is breathing is to notice their shoulders," he said. "With more rapid breathing that's higher up in the chest, you will see their shoulders rise and fall a lot more than normal." When coaching executives to improve their negotiation skills, Moine finds that many of them miss opportunities to read others' body language. "They often make the mistake of looking down at papers instead of being observant," he said. The next time your opponent gestures for you to study a document or presents a written contract for you to scrutinize, don't fall for the bait.

Instead of cutting off eye contact, Moine suggests that you say, "Tell me about it. What does it say?" As long as you keep your eyes on your negotiating partners, you can assess whether their nonverbal behavior conflicts with what they tell you.

Even the most experienced deal makers who know how to mask their expressions may still betray themselves with their bodies. "Watch for signs of deception," said Raymond McGraime, author of "Silent Seduction." "Deception is shown by such movements as covering of the mouth with the hands, rubbing the side of the nose, jerking the head quickly to the side, and leaning away from you. If these things occur when they're saying something critical to the negotiation, that's even more significant." Although these behaviors may simply result from nervous tics, McGraime warns that they can also expose a liar.

"When most people lie, they subconsciously want to apologize for it," he said. "They feel guilty for lying, and that shows in their nonverbal behavior." On a more positive note, body language can sometimes help you trust a speaker. Look for expansive, welcoming gestures that seem to flow naturally from the person's behavior. "When someone opens his palms towards simultaneously, that's a sign of openness and honesty," McGraime said. "The further the palms come out from their body, the better. In depictions of the great prophets, you see this. It's like saying, 'I have nothing up my sleeve.'"

Now that you know what to look for while negotiating, beware of attributing too much meaning to every little move your opponent makes. Resist drawing rash conclusions based on someone who suddenly starts scratching or acting jittery. "The danger of reading your opponent is that you lean too much on just one sign," said Richard Heslin, professor of psychology at Purdue University in West Lafayette, Ind. "But when you can put several things together, maybe there's something there that's worth paying attention to."