What questions have been useful or not been useful?
The first question I always as when interviewing dev candidates is have they ever worked with a designer before? From there I ask a lot of questions about process. If they've worked with a designer before, what was their process, what did they like/dislike about it. Regardless if they've worked with a designer before, I ask about what their expectations are. Do they want input into the design? Do they expect the designer to handle slicing and optimizing of images, or anything else beyond just handing over mock-ups. Do they have an ideal process? Basically, I want to figure out if their expectations and my expectations match up, and if not, is there room for compromise. The main question I want answered is, can I work with this person.
Pose questions about the areas where you two will overlap. Or, if there is no overlap, pose questions about their skills in back/front-end development and even design. I would also throw in some questions to gauge personality as well - Since others may be primarily focused on skills, you can get a good understanding of them as a person instead.