Prior Incantato

''Well, we'll soon see,' growled Mr Diggory, looking unimpressed. 'There's a simple way of discovering the last spell a wand performed, elf, did you know that?'
Winky trembled and shook her head frantically, her ears flapping, as Mr Diggory raised his own wand again, and placed it tip to tip with Harry's.
'Prior Incantato!' roared Mr Diggory.

THE DARK MARK - Pg 115