Sergey and swilliams both gave great answers, in particular, swilliams mention of asking for a portfolio is key. With a portfolio you can also test for items like,
- does the html and css validate?
- does the presentation render consistent across browsers?
- does the candidate have JavaScript errors? if they do, does the person let them bubble up to the presentation layer or do they at least catch them with a try/catch block?
- in terms of JS, how advanced is the person? Can they do form validation? Can they do regex? Are they relying on MM_Preloader? (Yuck!)
A portfolio can also give a sense of how passionate someone is about web development. Moreover, if they've done a site for someone else, that alone presents an opportunity to talk about a number of things with a candidate,
- how did they go about developing the UI?
- what kind of planning went into the site?
- how were user expectations uncovered/met?
- what kind of challenges during construction came into play?
Beyond these items, one other approach you might want to consider is a developer test that you could send a prospective hire. Nothing too hard that would take more than a day, but enough of a brain teaser to see if they can work through a CSS or JS problem.
