Several good responses from Quora.
We run potential candidates through a fictional design exercise—one that has nothing to do with any project we’ve worked on or intend to work on, but that we can assess the candidates ability to think through a design problem.
– Layout general: Why did you choose this layout? What alternatives did you consider. The point is to figure out how much thought went into deciding the layout; whether it was “instinct” or whether there were some cognitive/gestalt/other principles used. There are no “correct” answers – but the candidate needs to be able to talk intelligently about choices and articulate the decisions.
– Layout specific: What would happen to the design if I shifted this element here? Would it be worse? Why?
– Info Design: Ask about how he/she chose what should be on the screen and what shouldn’t. Was a process followed? Were users consulted? Were alternatives tested with users? Why or Why Not? Basically you’re looking to see if they just crammed in as much functionality as possible based on gut feel, or whether there was prioritization based on some reasonable/reproducible process.