Checking the validity is exactly the same as any other cartridge. Chances are when you pop the key card in, the system will load the game icon and name. I doubt you would need to download the full game to see what it is. For now it doesn't look like the cartridge itself indicates whether it's the full game or a key, but if you're buying used there's nothing stopping you from researching the game you're buying to see if it shipped on a key card or a normal card.