Dark Detectors are objects than can be used to detect lying, concealment, or even when Dark wizards or one's enemies are around. However, they should not be relied on too much, as they can be fooled.
Examples of some Dark Detectors are:
When Dumbledore's Army was formed in 1995 there were various Dark Detectors in the Room of Requirement when it served as the D.A.'s Headquarters.