Google has released a video named how to hire an SEO and Maile Ohye, a Googler part of the Google Search team for 10 or so years, is the face behind that video. The video itself goes into detail on how to hire an SEO, including what to watch out for.
The SEO hiring process according to Google is three part:
(1) Conduct a two-way interview with your potential SEO. Check that they seem genuinely interested in you and your business.
(2) Check their references.
(3) Ask for a technical and search audit.
Here is the video, which is something you should watch (or in this case, just listen to).
Do you guys disagree with anything said in the video?
Forum discussion at Google+.