A question like this is really to general to be answerable. To be able to qualify a person to handle SEO for you, you need to understand SEO principles yourself. Then, ask the person you are about to hire questions based on your own knowledge of the answers.
It would depend on whether you were hiring a full-time in-house SEO (an employee) or a consultant/agency to which you wish to outsource your SEO. It also depends on whether you’re looking to rank in local listings for local goods/services or wanting to compete nationally or internationally. These are very different skillsets.
When inviting candidates for an interview ask them to have a look at your website with their “SEO-glasses” on beforehand also ask them to bring/prepare examples of SEO projects they have done before and how these performed and how they could relate to the present job situation/your websites. As a minimum, check their references on LinkedIn and call those persons listed as references. Nothing beats a one-on-one conversation on the phone about a potential employee with his/hers previous employers.
We believe the questions you need to ask should be centralized around your SEO goals and how this person plans to accomplish these goals. We assume that because you are part of this group, you have a least the very basic knowledge of SEO in general, so ultimately, we would wait to hear how this person responses to my SEO goal then decide who is a better fit In our opinion, if you are not extremely knowledgeable about SEO, your best approach to interviewing a potential SEO candidate is to pay someone with a lot of SEO experience to interview them for you. Otherwise, the candidate might “seem” impressive while all the while blowing smoke.