The first thing to make clear is that body language and facial expressions are good indicators of when a person is lying but the indicators are not foolproof. These indicators only give you a "heads up warning" that something is not quite right about what the person is saying.
To detect lying, watch for signs of stress in body language and facial expressions. When most normal people (not psychotic) are lying they show signs of stress in their posture, movements and facial expressions. However, their stress may not show up if the person really wishes the facts were as described or they can rationalize that their story really should be as described. But generally, lying people will act and move in slightly different stressful ways from when they are telling the truth. They also delay a few seconds longer when answering a question with an answer that is not truthful. Truthful answers come sooner then untrue answers.
Here is what to watch for when a person is lying to you when face-to-face: (Also make sure you don't do these things!)
Keep in mind that periodically everyone tells "little fibs". That is what we call lying when it serves us: fibs. In many cases we lie to protect a person's feelings, such as: "You look so much thinner since you cut down to only 5 hamburgers a week." We also lie periodically to avoid doing something we don't want to do, such as: "I can't go shopping with you because I have to wait for a phone call." We also lie to increase our importance, such as: "I could have won the award but I didn't try so the little girl could win."
One researcher, Joseph Tecce at Boston College, has identified six types of lies:
Lying occurs frequently in the flirting and dating game. It happens often because telling the truth may block the liar from getting what he or she wants, but a little lying may facilitate getting what is wanted.
Women often lie to make others feel good. This may increase the relationship, which is something of primary importance to most women. Men often lie to make themselves look good and increase their status, which is something of primary importance to most men.
Both women's and men's lies are often used to elevate their status to discourage competition. An example is when a woman says to a female friend who shows interest in her boyfriend: "My boyfriend is impotent, snores and really is a slob, so I just go with him to help him."
Another thing to watch for to detect lying is a sudden change in movements. The liar tends to shut down and tries to maintain control of the situation and in so doing becomes quieter and stops normal body movements until you have accepted the lie. A liar may leave out pronouns and speak in a monotonous tone. This heighten tension may cause the eyes to increase blink rate also. Any spoken words during and immediately after the lie will come harder and there may be more than normal mispronunciations and stutters. The liar is more defensive than usual and also may place objects (cups, keys, pencils, chairs etc.) between self and others.
Since tension is high in liars, they need some self-comforting. They stroke their hair and touch their face more frequently and harder than usual. Scratching and rubbing their nose is common in liars, but don't accuse all nose-rubbers of being liars! The best overall liar detection clue is a sudden change in posture and movements from the normal patterns for a short time until you have accepted what is said. If you believe someone is lying, change the subject quickly and watch their reactions. A liar will follow along willingly and become more relaxed. The guilty wants the subject changed but an innocent person may be confused by the sudden change and will want to go back to the previous subject.
The Evolutionary Roots of Deception and the Unconscious Mind" is a fascinating book about this subject of lying. The author, David Livingstone Smith, believes that lying (and other deceptive traits as well) are deeply embedded in our subconscious as a result of evolution. This means simply that our many ancestors who survived by lying (and doing deceptive things) passed on stronger and stronger genes in each generation for this talent. The ancestors who did not have the knack to lie and deceive died off. Thus, evolution produced the best liars and deceivers. Interesting thought.