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In some situations, this becomes a Blessed with Suck power.
The character can't disable it, so they know whenever anyone is lying to them, driving the character nearly insane from all the lies.
It can also be a learned ability, making this Truth in Television.
This is because most people fidget, look away or do something else when they lie.
So if a character is a Living Lie Detector, (s)he could just know what to look for.