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Ekman reveals that when looking for signs someone is lying, there are two clues to deceit you can reference: leakage, and deception clues. "When a liar mistakenly reveals the truth, we call it leakage," he writes. "When the liar’s behavior suggests he or she is lying without telling the truth, we call it a deception clue."

Before trying to figure out whether someone is lying, you need to first establishing a “baseline,” or a behavior considered to be “normal” for the individual in question, says Ekman. "The basic process of behavioral deception detection then involves looking for clues that signal some kind of change or deviation from this baseline," he adds.

Ekman says a facial expression, tone of voice, slip of the tongue, or certain types of gestures could leak the person's true thoughts or feelings, "as long as it’s measured against the baseline."

But he adds that "while a deception clue can tell you if someone might be lying, it only answers the question of whether or not the person is lying; it does not reveal what is being concealed."

Only leakage would do that, he reveals. "Often it does not matter. When the question is whether or not a person is lying, rather than what is being concealed, a deception clue is good enough. Leakage is not needed. What information is being held back can be figured out or is irrelevant."

Someone's face often contains two messages when lying: what the liar wants to show, and what the liar wants to conceal. "Often, these hidden emotions leak in the form of a micro expression, a brief (half a second or less) involuntary facial expression revealing true emotion," writes Ekman.

He adds that while a single micro expression or flash of leakage does not offer conclusive proof of lying, "micro expressions are one of the most effective nonverbal behaviors to monitor to indicate a person is lying."