I like a variant of Ben's question: pick a game they really love, then ask what they would change about it. If the answer is "nothing, it's perfect in every way"... well, that's how you tell the difference between a gamer and a designer :)
For level designers, you could always ask them to, you know, design a level. Or assuming you have them submit level design samples with their application, have them walk through their level explaining the choices they made. Critique their level as you go, to see if they can defend their choices while remaining polite when they know they're right (and to see if they can take constructive criticism when they're actually wrong).