I tend to think that a portfolio is the best way to get a feel for a designer's abilities. It's not fool-proof, but it gives a good overview in most cases. Th

I tend to think that a portfolio is the best way to get a feel for a designer's abilities. It's not fool-proof, but it gives a good overview in most cases. There are quick tests that are obvious with a portfolio: does it look good, does it make sense to you. Beyond that, you could take some of their work and conduct usability tests if you, yourself cannot identify problems in that area. Basic usability tests will give you a good idea of how well the designer understands it. Another trait that you should watch for is variety. Make sure they can demonstrate proficiency in a number of design styles (unless your needs are particular and they meet that area). As for interview questions, you can get a good idea for usability instincts by asking for on the fly designs for simple interfaces. If you have the budget, the best of the best are household names (or you can easily look them up). These will be sure bets to get you a great design, but it will cost. Something to consider when searching is to find a similarly structured application to yours, with a design you love, and find out who did it.