Know how he talks normally. The first step to reading a liar's body language is to know his normal, everyday conversational habits. This is referred to as someone's “baseline” behavior – normal eye, hand and body movements during a pleasant conversation in which there is no pressure or lying. This will allow you to detect variations when the lie occurs, without mistaking a naturally awkward or shy person for a chronic liar.
Notice how stress influences his patterns. It is notoriously difficult to tell the difference between a person who has been put on the spot (that is, under stress and pressure), and an out-and-out liar. This is because most of the tell-tale signs of lying are based around the fact that telling a lie is a stressful event on the body and mind – it seems humans are not built for distorting the truth, and forcing that situation has recognizable repercussions.
Look for the signs. This is a list of the most common “tells” that people have while attempting to lie. Again, note that most of these also occur when someone is feeling stressed by the conversation.
Little to no body movement. People will sometimes “freeze,” not quite facing the other person, and will move as little as possible during the lie. Alternatively, some people will fight this by being overly dramatic, moving the body much more than normal to try and “sell” the lie.
No eye contact. A lot of information is conveyed through eye contact, and most people have an extremely hard time lying to someone while looking directly into his eyes. Sometimes this will only occur at the moment of the lie, a brief glance at the floor or to the right. Again, some will try to sell the lie by making and maintaining eye contact fiercely, much more so than usual.
Stress gestures (rapid blinking, scratching, itching, swallowing, fidgeting, etc.). If discovered, a deception carries much greater risk of punishment than simply telling the truth, and people understand this. This uncomfortable situation will make them uncomfortable, and they will act accordingly.
Looking up to the right. Looking up and to the right stimulates the part of the brain associated with imagination (that is, making things up), whereas looking up and to the left stimulates the part of the brain associated with recalling memories (that is, telling the truth).
Touching the nose and covering the face/mouth. Habits from childhood, these gestures are a subconscious attempt to “cover up” the lie, and put distance between the liar and you.
Rise in vocal pitch. Another product of stress, the liar's voice will get higher and squeaky. This should be noticeable if you are familiar with his normal speaking voice.
Wide-eyed, innocent look. Another product of childhood fibs, based on the “who, me?” fake innocence usually associated with a kid denying he has his hand in the cookie jar while it is still in there.
A noticeable pause in speaking. Most people lack the ability to come up with a plausible fiction in an instant, so they have to take a moment to think something up. This results in a pause, usually right before the lie itself, while they desperately reach for something other than the truth. Careful questioning will reveal variations in their story, as a consistent fiction is hard to remember, whereas actual events are usually easy to recall.
Lying by omission. Sometimes a lie will happen by not speaking – that is, the liar omits an important piece of information from an otherwise truthful statement. However, this omission will, in the mind of the liar, poison the “truthful” statement, thus rendering it a lie. The above signs will still be present, with the addition of more obvious signs after he has finished talking. (An example: After talking in a relatively truthful manner, he falls silent and immediately looks away, covering his mouth and fidgeting.)
Ask. A risky maneuver, as a person who is telling the truth may be horribly offended. Sometimes, however, a liar will come clean if given the chance to, as he realizes that continuing to lie when you know he's doing so would be far worse than just admitting the lie.
Trust your intuition. Notice I say “intuition” and not “desire.” Most people have a pretty decent “lie detector” built in, and if you trust it, it can usually point the way. However, some people may want to believe they are being lied to, and they mistake this desire for “a gut feeling.” So before you start accusing anyone of a lie, make sure you're not trying to project your desire on an innocent person.
Using the above steps, you should be able to determine the difference between a lie and the truth to a much more accurate degree. And while we may not have an answer to how to define truth for certain, we will at least be able to tell if the other person feels like he is lying...and if it feels like a lie, it probably is.