5 subtle clues Mark Zuckerberg isn't as confident as he may seem in front of Congress

Hadnagy said this is one of the clearest signs (though it's still not a sure sign) that someone might be lying.

For example, if someone starts telling a story with the pronoun "I" — "I went here, I did this" — and switches suddenly to the pronoun "we" — "We drove there, we wanted this" — that could be a sign of something fishy.

Hadnagy gave an example of what he calls "incongruent body language."

Let's say Zuckerberg says, "I had no clue" that Facebook users' data was being leaked to Cambridge Analytica, but his head is nodding "yes" while he says that. That mismatch could potentially mean he's not telling the truth.

Hadnagy calls these "manipulators." When a person gets uncomfortable, they may start rubbing their hands together, cracking their knuckles, or rubbing the back of their neck.

This type of body language, Hadnagy said, "could indicate the discomfort level even though [Zuckerberg] may be trying to answer positively or answer with confidence."

Business Insider previously spoke to Joe Navarro, a former FBI agent and the author of "What Every BODY is saying," and he talked about the significance of pacifying, or soothing behaviors. Navarro cited rubbing the skin on your hands as a common one, noting that some people under tremendous stress end up rubbing their hands until they bleed.

This happens, Hadnagy said, when someone doesn't answer a question directly, instead telling a story that's only tangentially relevant. It gives the person "time to formulate the answer they want to give," he added.

"Eye blocking" occurs when someone blinks for too long or covers their eyes.

Researchers have found that eye blink rate increases when you're under stress, like when you're nervous or when you're lying about something. It's an automatic physiological response, so you might not even realize when it's happening.

Hadnagy also cited pursed lips as an indicator of discomfort and, potentially, dishonesty. Some people may place a finger or a pen on their lips.

David Givens, an expert in nonverbal communication, writes in his book "Crime Signals" that President Clinton's lips "visibly pressed together, tightened, and rolled into a thin line each time he talked with reporters about the Lewinsky affair."

According to Givens, we instinctively tighten our lips to protect ourselves from danger — real or imagined.