SEO Certification? There isn't any? Why? Because things change and developing any criteria would make those same criteria useless.
If you want to know how to hire an SEO.
Contact the Better Business Bureau.
Ask for references.
Look out for companies that will work with you on a month to month basis, SEO is a long-term activity and if you don't plan to be in business 5 years from now, don't engage in it.
Read what Google says to look for in a Organic Search Engine Optimization firm.
What's an SEO? Does Google recommend working with companies that offer to make my site Google-friendly?: "SEO is an acronym for 'search engine optimization' or 'search engine optimizer.' Deciding to hire an SEO is a big decision. Make sure to research the potential advantages as well as the damage that an irresponsible SEO can do to your site."