I always like to throw some "hypotheticals" into an interview for a UX designer, and here's a couple I sometimes use:
"You have two choices: Lead a high profile design project with a huge budget that has a low chance of being implemented or a small design project that will almost certainly be. What would you choose?" This question tells me if they are in it for the glory and fun of "just designing" or want to make a difference in people's lives.
"A key stakeholder says "this is crap" when you are presenting a design you and your team worked extremely hard to create. How do you respond?"This tells me how they will deal with criticism and how they will react to an uncomfortable business situation.