Muggles' Guide to Harry Potter/Magic/Prior Incantato

Overview
The Prior Incantato charm is used to reveal the last spell cast by a wand. Once the spell cast has been determined, the Deletrius charm can be used to end the display. The effects are similar to the Priori Incantatem wand effect, but not entirely related.

Extended Description
In Harry Potter and the Goblet of Fire, the House Elf Winky is found Stunned, holding a wand, shortly after the Dark Mark is cast. The Prior Incantato charm is used to determine that it was, in fact, the wand that was found with Winky that was used to cast the Dark Mark.

In Harry Potter and the Deathly Hallows, Hermione's wand is captured at Malfoy Manor. Harry assumes that, by means of the Prior Incantato charm, the Death Eaters there will be able to determine that Hermione had tried to repair Harry's wand, and had failed. Remote as this possibility may be, Harry decides that he must proceed as if Voldemort is aware that his wand has been destroyed.