When hiring, how do we assess whether someone is good at SEO when we dont know much about it ourselves?

We're looking to bring on an SEO subcontractor for our business, we're a digital agency, we're a web developer and a digital marketer (Google & FB ads), so we understand the basics of SEO, but honestly, not much more than that. We're now constantly getting asked by our client base to do SEO and we'd like to be able to provide that service via a sub-contractor rather than just saying no.

How do we assess if someone is good or not? Obviously from a business perspective, we need someone that is good at verbal and written communication, we need someone that will be able to produce initial assessments and monthly reports etc. So I'll ask to see previous examples of such. Is a good SEO person judged soley by "did you get X website to rank?" or is it about their strategies? Knowledge? Something else? Particular areas of knowledge?

I guess i'll get more of a feel for it when i start talking to people and compare them against each other. Should I give them an example of a client website and ask "What would you do?".

Suggestions welcome. Thank You.