This article on vooks.net tickled me so much I thought it deserved a mention. It revolves around a store, JB Hi-Fi store in Malvern, Victoria that has a helpful guide to clarify the difference between Wii and Wii U games with an in-store sign.
They’ve got a piece of cardboard and both written and drawn on it, Wii and Wii U games cases.
Wii = white case.
Wii U = blue case.
Simple but effective.
Hit the vooks.net link to view the image.