Assuming you have already decided that hiring is sensible, the selection problem is that everything a supplier shows you is chosen by them. Case studies survive because they worked, rankings can be for terms nobody searches, and testimonials are collected from happy clients. What discriminates is not the material they present but the answers they give to questions they did not prepare for, and there are only about five worth asking.
Ask what would make them turn this work down
A competent supplier has a view about which businesses they cannot help: the wrong economics, no capacity to implement, a market that is not winnable, an offer that is the real problem. An agency that would take any client is either very early or is selling a package. The answer also tells you whether they are going to be honest with you later, when the news is worse.
Ask for a piece of work that did not succeed
And what they concluded from it. Practitioners who have done this for years have several and will discuss them with specificity. The incompetent rarely have the vocabulary, and the dishonest will produce a story where the client was at fault. This single question separates more suppliers than the rest of a selection process combined.
Ask who does the work and meet them
Not the salesperson, the person who will write, fix and decide. Ask how many other clients they carry and what happens when two need something in the same week. In agencies of any size the gap between who sells and who delivers is the largest source of disappointment, and it is visible in the first meeting if you ask.
Ask what they need from you, in hours
Any real engagement needs access, subject-matter time, approvals and answers only you have. A proposal that asks for nothing describes work that will be written from your competitors' websites. A supplier who names the hours they need, and says what happens when you cannot supply them, has done this before and is telling you the truth about the effort.
Never accept a guarantee of rankings
Nobody controls a search engine's results, so a guaranteed position is either meaningless, for a phrase nobody searches, or a promise somebody intends to keep by means you would not approve of. This is the one hard rule in supplier selection. Everything else is judgement; this is a disqualification, however reasonable the rest of the proposal looks.
Questions people ask about how to find the right seo company
What is the single best question to ask an SEO agency?
Describe a piece of work that did not succeed and what you concluded. Experienced practitioners answer with specificity; the inexperienced lack the vocabulary and the dishonest blame the client.
Are case studies useful?
Only mildly, because the supplier chose which ones survive. More useful is asking what they would decline, what they need from you in hours, and who specifically would do the work.
Should we ask for a guarantee?
No, and you should refuse one if offered. Nobody controls a search engine's results, so a guaranteed ranking is either for a phrase nobody searches or a promise kept by means you would not approve.
How do we compare proposals that differ completely?
Write the scope yourself first, even briefly: what you want changed, by when, with what access and what you will supply. Then ask each supplier to price that. Otherwise you are comparing three different projects.