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It may be the natural ability of an alien or some other non-human species, it may be a super power specific to one human, or it may be the effect of some Applied Phlebotinum.Could also be a prominent fringe benefit of a broader power like telepathy.
An exceptionally self possessed and clever liar can sometimes be unreadable or even fool the Living Lie Detector.
Another is a threat of punishment, if you're not worried about consequences then you're either not afraid of the truth or you have nothing to lose.
Characters with this power are also highly likely to see through or uncover The Masquerade because of the power. See also Pinocchio Nose, You Can Always Tell a Liar, The Tell.
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.
Some characters get the added bonus of being able to people to tell the truth (as opposed to just remaining silent) rather than just detect deception.