How do you tell or know if someone is lying? Very Carefully. All methods of lie-detecting, including polygraph machines, are regularly ‘tricked’ or used improperly, resulting in false assumptions or mistakes. There are a few good accurate ways to tell, but try to confirm the truth by other means as well, such as confession or a background 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 …