1. They change their head position quickly.
If you see someone suddenly make a head movement when you ask them a direct question, they may be lying to you about something.
"The head will be retracted or jerked back, bowed down, or cocked or tilted to the side," writes Glass. This will often happen right before the person is expected to respond to a question.
2. Their breathing changes.
When someone is lying to you, they may begin to breathe heavily, Glass says. "It's a reflex action."
When their breathing changes, their shoulders will rise and their voice may get shallow, she adds. "In essence, they are out of breath because their heart rate and blood flow change. Your body experiences these types of changes when you're nervous and feeling tense — when you lie."
3. They stand very still.
It's common knowledge that people fidget when they get nervous, but Glass says that you should also watch out for people who are not moving at all.
"This may be a sign of the primitive neurological 'fight,' rather than the 'flight,' response, as the body positions and readies itself for possible confrontation," says Glass. "When you speak and engage in normal conversation, it is natural to move your body around in subtle, relaxed, and, for the most part, unconscious movements. So if you observe a rigid, catatonic stance devoid of movement, it is often a huge warning sign that something is off."
4. They repeat words or phrases.
This happens because they're trying to convince you, and themselves, of something, she says. "They're trying to validate the lie in their mind." For example, he or she may say: "I didn't ...I didn't ..." over and over again, Glass says.
The repetition is also a way to buy themselves time as they attempt to gather their thoughts, she adds.
5. They instinctively cover vulnerable body parts.
This may include areas such as the throat, chest, neck, head, or abdomen.
"I have often seen this in the courtroom when I work as a consultant for attorneys. I can always tell when someone's testimony has hit a nerve with the defendant, when I see his or her hand covering the front of his/her throat," says Glass.
"I never appreciated the potential use of this very telling behavior until I joined the FBI as a Special Agent," she says.
6. They touch or cover their mouth.
"A telltale sign of lying is that a person will automatically put their hands over their mouth when they don't want to deal with an issue or answer a question," says Glass.
"When adults put their hands over their lips, it means they aren't revealing everything, and they just don't want to tell the truth," she says. "They are literally closing off communication."
7. They shuffle their feet.
"This is the body taking over," Glass explains. Shuffling feet tells you that the potential liar is uncomfortable and nervous. It also shows you that he or she wants to leave the situation; they want to walk away, she says.
"This is one of the key ways to detect a liar. Just look at their feet and you can tell a lot."
8. They provide too much information.
"When someone goes on and on and gives you too much information — information that is not requested and especially an excess of details — there is a very high probability that he or she is not telling you the truth," writes Glass. "Liars often talk a lot because they are hoping that, with all their talking and seeming openness, others will believe them."
9. They tend to point a lot.
"When a liar becomes hostile or defensive, he is attempting to turn the tables on you," says Glass. The liar will get hostile because he is angry that you've discovered his lies, which may result in a lot of pointing.
10. It becomes difficult for them to speak.
"If you ever watch the videotaped interrogation of a suspect who is guilty, you will often observe that it becomes more and more difficult for her to speak," writes Glass. "This occurs because the automatic nervous system decreases salivary flow during times of stress, which of course dries out the mucous membranes of the mouth."
Other signs to watch out for include sudden lip biting or pursed lips.
11. They stare at you without blinking much.
When people lie, it's common that they break eye contact, but the liar could go the extra mile to maintain eye contact in attempt to control and manipulate you.
"[Bernie] Madoff, like most con men, overcompensated and stared at people longer than usual, often without blinking at regular intervals," says Glass. "When people tell the truth, most will occasionally shift their eyes around and may even look away from time to time. Liars, on the other hand, will use a cold, steady gaze to intimidate and control."
Also watch out for rapid blinking.
12. They buy time before answering a question.
When someone isn't being honest and you ask follow-up questions, they often try to stall on responding.
"They're trying to think of what to say next," says Glass.
13. They begin to perspire.
When people lie, they tend to get nervous and uncomfortable. And when you're nervous and uncomfortable, you tend to sweat.
"They may develop a bead of sweat over their upper lip or on their forehead," Glass warns. "This happens because their autonomic nervous system is working overtime."
