A test in which you provide in order to tell someone if something is clean as claimed by proving it by either telling them to eat off the plate from which they claim it is clean and it obviously is not or to tell them to drink out of a cup if they said it is clean . It is mostly used on kids but can apply to adults. This may also apply to anything that someone says is true but you know they are wrong so you tell them to prove it so it can blow up in their face.
Mom: Did you wash your hands to go eat? Hmmm…
Kid: Yes ma
Mom: Okay then lick your fingers, it’s a truth test
Kid: What? That’s nasty!!! I mean…better use water AND soap I guess