Just pull it out and shake it. If it doesn't rattle, it's failed.

It looks similar to this:



Why a failed PCV will cause oil on the filter of a stock airbox: When the valve sticks closed, the crankcase no longer vents through the valve into the intake manifold. Instead, it vents into the airbox upstream of the air filter via that hose that runs from the valve cover to the air cleaner housing.

 

^^ Gotcha.