A few of my own guidelines for hiring a search engine optimization consultant.
1. Verify a money back guarantee.
2. Discuss in depth the tactics the consultant uses to achieve rankings.
3. Talk to former customers and verify references.
4. If the SEO consultant is a member of the Better Business Bureau, check their standing.
5. Are they listed in top web directories like DMOZ, V7N and Yahoo?
6. Legitimate SEM companies have toll free numbers that they answer.
7. Ask about their tactics - if they do not include link building such as web directory submission you might as well just flush money down the toilet.
Some of the best places to find a qualified SEO Consultant include the Search Engine Optimization Consultant Directory ay V7N, posting on SEO Forums, or posting on an SEO job board.
I would recommend visiting the Seattle SEO consultants page on V7 Network, or the New York SEO consultants page, or the Chicago SEO consultant page. The most qualified search engine ranking advisors can be found on those pages.