The supposed "tells"—looking off to the right, body language, nervousness, "microexpressions"—fall apart under scrutiny.
Study after study has found that attempts – even by trained police officers – to read lies from body language and facial expressions are more often little better than chance. According to one study, just 50 out of 20,000 people managed to make a correct judgement with more than 80% accuracy. Most people might as well just flip a coin.
Polygraph machines and other "lie detectors" are garbage. They're pseudoscientific intimidation tools and widely inadmissible as evidence.
What works, as any novelist knows: make them talk and watch for change.
Use open questions. This forces the liar to expand on their tale until they become entrapped in their own web of deceit.
Employ the element of surprise. Investigators should try to increase the liar's "cognitive load" – such as by asking them unanticipated questions …
Watch for small, verifiable details. If a passenger says they are at the University of Oxford, ask them to tell you about their journey to work…
Observe changes in confidence. Watch carefully to see how a potential liar's style changes when they are challenged…
Also, don't tip 'em off when you figure them out. Just let them keep on rolling.
