How to tell if someone is lying? Carefully. All methods of lie-detecting, including polygraph machines, are regularly fooled o used improperly, resulting in mistakes. There are a few good ways described here, but try to confirmar the truth por other means as well, such as confession o investigation of the facts.
You should compare behavior in the person under suspicion to their "base" behavior, if possible. In other words, if they always have shifty eyes, then this trait can't be considered as an indication of lying. On the other hand, if they are normally very calm under pressure, but start...
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