Hello! I am interviewing candidates for a role that centres around user experience. The issue is that I am unsure of the best way in which I can test someones ability in this field (i'm used to grilling them with OOP questions for dev roles).
I had thought of giving the candidate a PSD file and seeing if they can translate it into compliant xhtml and css etc.
I have never been a fan of programming tests being used to determine the skill of a developer.
Unless you're hiring an entry-level developer, you should probably judge them more on their portfolio and have them give you some source to things they've worked on. High quality, well-designed code takes time to produce - and it won't happen during a test except on a very small scale. The test should be more of a formality, so you know they are not lying about all the stuff they developed.
Show them some markup and code with a few obvious and not so obvious mistakes and see how many they spot. That way you know their brain is working in the right way when it comes to examining and evaluating it.
Related to Meanstreak's answer:
Take a look at jQuery Tips and Tricks and jQuery pitfalls to avoid.
Create some mock-up code - implementing the "dont's" - and ask the interviewee to optimize it.