They make you add a new sticker to the license plate with an expiration date and new color or other visual marker. This is to make it obvious whether taxes have been paid. But now they just have automatic license plate readers for that, (and can also log vehicle locations and look for warrants, etc). It isn't or wasn't enough because these systems are archaic and poorly designed (as I once found out after being pulled over at gunpoint for a DMV mistake).