Rob Walling writes about Personality Traits of the Best Software Developers. If you're hiring a developer, here are some questions you should ask (paraphrased from Rob's article): Are you an optimist or a pessimist? If they are pessimistic in the short term (the software's going to have lots of bugs) and optimistic in the long term (but the software will be perfect) then they're on your short list.
Tell me about a time when you found the source of a problem. Most people find ways around problems, but don't identify the source. Great developers identify the source.
Do you save for retirement? Rob suggests you work this in during discussions of your company's retirement plan. If they do plan for the future of their lives, you can bet they'll be able to plan for the future of the software.
Can you see anything wrong with this code? Make an obvious misspelling in a short code sample and ask if they see anything wrong - the best developers have an acute eye for detail and will cringe at $user->getEmailAddrres()!