Aaron aka TheMadHat wrote a post about SEO interview questions. If you're hiring an SEO, you might want these questions to be on your interview list. Some of them include talking about your general experience or search conferences that you've attended, your familiarity with black-hat techniques, thoughts on competitive analysis, and more. The post is definitely a good place to start if you're seeking to hire some SEO professionals, and the forum discussion at Sphinn pretty much is in agreement.
Still, as Tim Dineen mentions, most of the folks that you'd want to interview who can answer these questions are probably not available for hire. But Jill Whalen says that all hope isn't lost. Find somebody new and train them.
Those questions are fine if you're looking for someone with some SEO experience. I've found that you might be better off looking for someone without SEO knowledge, however, and then training them.
True that. Sometimes that can be risky because they can either work for you or become your competition. There's risk involved in any such area, and this is no exception.
Forum discussion continues at Sphinn.