Looking for reviews of an agency is sensible and the supply is bad. Most agency directory listings are paid placements, the reviews on them are collected by the agency from clients it chose, and the outcome being reviewed is one almost no client can independently verify. That is not a conspiracy, it is how the format works, and it means the time is better spent on questions than on stars.
Understand what an agency review actually is
Usually a happy client, approached by the agency, describing an experience rather than an outcome, on a directory where position is influenced by payment. That can still be informative about responsiveness and communication, which is worth something. It is close to useless about whether the work produced results, because the client rarely has an independent way to know.
References are better than reviews, and you can choose them
Ask for two clients in your situation and one that ended. The last is the informative call and the reaction to the request tells you a great deal. Speaking to somebody who left, and hearing a measured account of why, is worth more than any number of five-star entries collected by the agency itself.
What the agency refuses to do is the best signal available
Ask what work they decline, what they would tell a client who should not be spending, and about a project that did not succeed and what they concluded. Experienced practitioners answer all three with specifics. It is not a review, it cannot be gamed, and it separates suppliers faster than anything published about them.
Look at their own clients' sites, not their case studies
Pick three clients from their portfolio, look at those sites now, and search for the things those businesses would want to be found for. It is an hour and it shows you the standard of the work as it actually exists rather than as it was presented. Case studies survive because they worked; the portfolio is the fuller picture.
The one hard rule is not a review question
Never accept a guarantee of rankings, whatever the reviews say. Nobody controls a search engine's results, so a guaranteed position is either meaningless or a promise somebody intends to keep by means you would not approve of. A glowing review profile does not change that; it usually means nobody has been penalised yet.
Questions people ask about small business SEO reviews
Are agency reviews trustworthy?
Rarely, because most directories are pay-to-play, the reviews are collected by the agency from clients it chose, and the client usually cannot independently verify whether the work produced results.
What should we do instead?
Ask for references including a client that ended, ask what work they decline and about a project that failed, and look at three of their current clients' sites yourself.
Is a review with details more reliable?
More informative about communication and responsiveness, which matters. Still not evidence about outcomes, because outcomes in this trade are hard for a client to attribute.
What is the one thing to refuse outright?
A guarantee of rankings, whatever the review profile says. Nobody controls a search engine's results, so the promise is either empty or intended to be kept by means you would not approve.