The same question you would ask in any other job interview works perfectly well when hiring a service designer.

To me, the best way to evaluate skills can be done with some of these techniques:

Ask the person to describe details about on the tiny part of the process or tools she used.

For example, take a real challenge you had at work and that you have fixed already and give that challenge to the person in the interview. Ask her how she would fix that problem. Another example of a challenge could be to ask the person to make a "time" budget for a service design project.

Here, the service designer has to create a plan for a project and estimate how much time he'll use for each part. Then, give the same challenge but with half of the time, and see how the person deals with constraints.