For shame, should've played that better, it litterally took me 5 seconds to find this:
Summary
Each Night the Lie Detector sends the Moderator a post or statement made by another player, and the Moderator tells the Lie Detector whether that other player was telling the truth when they made that post.
A sample use of this power would be to find out whether a role claim is true or not. However, trying to use this ability on something that cannot be verified objectively (i.e. whether someone was telling the truth when they said that a wall of text was a waste of time) the moderator will likely just say "I don't know".
Lie Detectors tend to be Town, and also tend to be X-Shot.
Town-aligned Lie Detectors can easily translate themselves into Cops simply by forcing each player to say "I am aligned with the Town". Thus, they are not seen often at all.