Ask a real problem that is troubling you. The answer you get need not be the best solution possible, but you should be skilled enough to find holes in the solution. See how much of the solution provided, is something they can defend. Also check how much open they are open to changes. Too flexible is not good. Too rigid is not good either. When the decision is based on data, it is best. Finally ask them to walk through their favorite project and explain what they contribute, how it impacted the project and how they worked with the rest of the team. Hiring a good designer is not about hiring the smartest guy but about making sure you can get somebody who is an excellent team player as the role will be the glue between multiple departments...