It's conventional wisdom that you can detect lies by watching for signs of nervousness: sweating, blinking, eye contact, etc. However, apparently it's much more reliable to watch for signs that your quarry is thinking hard: signs of cognitive load.

Lying can be cognitively demanding. You must suppress the truth and construct a falsehood that is plausible on its face and does not contradict anything known by the listener, nor likely to be known. You must tell it in a convincing way and you must remember the story. This usually takes time and concentration, both of which may give off secondary cues and reduce performance on simultaneous tasks.

When nervous we blink our eyes more often, but we blink less under increasing cognitive load (for example when solving arithmetic problems). Recent studies of deception suggest that we blink less when deceiving -- that is, cognitive load rules. Nervousness makes us fidget more, but cognitive load has the opposite effect. Again, contra-usual expectation, people often fidget less in deceptive situations. And consistent with cognitive load effects, men use fewer hand gestures while deceiving and both sexes often employ longer pauses when speaking deceptively.

[The Folly of Fools: The Logic of Deceit and Self-Deception in Human Life]

It's also worth reading about red flags that police detectives use to identify liars.

When questioned, deceptive people generally want to say as little as possible.

Although deceptive people do not say much, they tend to spontaneously give a justification for what little they are saying, without being prompted.

They tend to repeat questions before answering them, perhaps to give themselves time to concoct an answer.

They often monitor the listener's reaction to what they are saying.

They often initially slow down their speech because they have to create their story and monitor your reaction, and when they have it straight "will spew it out faster," Geiselman said.

They tend to use sentence fragments more frequently than truthful people; often, they will start an answer, back up and not complete the sentence.

They are more likely to press their lips when asked a sensitive question and are more likely to play with their hair or engage in other "grooming" behaviors. Gesturing toward one's self with the hands tends to be a sign of deception; gesturing outwardly is not.
Truthful people, if challenged about details, will often deny that they are lying and explain even more, while deceptive people generally will not provide more specifics.

When asked a difficult question, truthful people will often look away because the question requires concentration, while dishonest people will look away only briefly, if at all, unless it is a question that should require intense concentration.

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