Sort of following up on some other answers here, I would stay clear of looking for a consultant via Google searches. Stick to referrals and social media. In finding a good SEO it's all about trust. Since so many can not be trusted, it's important to vet them in someway.
As far as referrals go here are some tips;
- Look up the reputable conferences such as Mozcon, SMX, SearchLove etc - look at the speaker list and start by contacting them. They will likely be too busy but they should be able to refer you to someone.
- If you know any designers, developers, CRO, UX people - ask them.
For social media, what I mean by this is - start looking around Twitter and Google Plus for SEO's that seem to have a good following, share valuable content, have a good attitude, and are generally helpful. Follow some people for 2-3 months to really get to "know" them.
I would also look at the helpfulness and quality of content published by SEO's. Their blog posts and articles will generally be indicative of the quality of work they do.
Ask for anyone you may want to contact to provide referrals of clients that you can pick up the phone and talk to.
To be honest, as an SEO consultant myself, I can attest that the WORST leads come from ... ironically ... SEO (someone Googling "seo services" and then emailing me). I wish I didn't rank #1 for "SEO Massachusetts" because this has only been a source of poor leads.
Finally, as others have mentioned Clarity does have some of the best SEO's on their netork. For example I can attest to knowing John Doherty and Nick Eubanks personally and their work and professionalism is top notch. Just look up any of their blog posts and you will see what I mean :)