Popular body-language claims ('crossed arms means defensiveness', 'avoiding eye contact means lying') wildly overstate what a single gesture can reliably tell you — most nonverbal signals are ambiguous in isolation and highly dependent on baseline, culture, and context. The more defensible approach is looking for clusters of signals that change together from someone's baseline behavior, not decoding any single gesture as if it were a fixed dictionary entry.
The most reliable nonverbal information isn't a specific gesture at all — it's a shift from someone's normal pattern. Someone who's usually animated going quiet and still, or someone who's usually reserved becoming noticeably fidgety, is more informative than any single universal gesture, because it's compared against their own baseline rather than a generic rulebook.
The Reid Technique, developed in the 1940s–50s and still taught to police departments today, instructs interrogators to read specific behaviors — crossed arms, avoided eye contact, shifting in the chair — as signs a suspect is lying. Psychologist Saul Kassin's research, including detailed analysis of proven false-confession cases (people later exonerated by DNA evidence after confessing to crimes they didn't commit), found that innocent suspects display these exact "deceptive" behaviors constantly, simply because police interrogation is stressful for innocent and guilty people alike. Trained interrogators using the technique were not meaningfully better than untrained people at telling true denials from false ones — but they gained false confidence from the training, which in documented cases led them to push harder and longer on suspects who turned out to be innocent.