I'm biased toward Dan's answer, being one of the SEO's in the Clarity network. ;-) However, I'm not currently taking SEO projects, so I can speak freely here from nearly 20 years of experience in the field.
The answer to your question has many layers, and can easily head you in the direction of a snake-oil salesman without some additional clarity.
As one example: Why SEO? Often the term SEO is used by "buyers" to mean something much broader than the "seller"/technicians view of "search engine optimization", creating an opportunity for things to go sideways from the very beginning. For instance, business owners frequently came to us seeking better visibility in search engines with an end goal of attracting more businesses - though it was fairly common to field requests for the technique du jour (SEO, PPC, Social Media) vs. the end goal. The thing is, the most cost-effective route to achieve the end goal might may not be the technique du jour. Propose the end goal, and let the experts map out a way for you to get there with the best ROI.
Jami's answer suggesting you ask for referrals is a good one. Candidly, I'd put more emphasis on referrals and results they've gotten for clients than personal SEO. Find someone who has created the value you want for others. Too many "games" go on for positioning of industry terms, which just creates another opportunity to wind up buying snake oil.
Another option is to attend search engine marketing events like SES or SMX. The type of person or company who invests in their professional education at events like these is a good start at finding some reputable help.
Happy to chat through what channels are likely to generate the best ROI for your business - feel free to give me a call. Good luck!