You are absolutely right, @JohnRobert, and it is the same problem with many other kinds of professionals, from your eye doctor or veterinarian to your tax accountant. You are at the mercy of the person’s own self-assessment unless you know his reputation. And the person who exhibits the greatest confidence in his skills may not be the one who ought to. It may come down to your intuition and your sense of whether you want to work with the person.
Many a reasonably good writer has been misdirected, damaged, or traumatized by a bad editing experience. People hire themselves out as writers, too, who have no business saying they can write. You can tell some things by asking questions (including whether the writer’s work has been edited and how the writer feels about working with an editor). But it is really hard to assess a sample unless you can take the time to read it carefully, and it is also pointless unless you know for sure that the work is original—and you are a qualified judge.
If I show you a heavily edited piece, maybe you can see my editing style. But a real editor knows when to do a light edit and touch only what needs to be touched. Does that mean it took less time or was an easier job than the heavy markup? Not at all. But it will be harder for you to evaluate my work. Unfortunately for the hiring process, it is the handling of the best writing, not the poorest, that really shows an editor’s art.
I am a rigorous interviewer of both writers and editors, and I wouldn’t hire either without some kind of on-the-spot test. For one thing, I always pose a tough grammar question and ask the candidate to think aloud while answering it. I care less about the correct answer (which can be taught) than about the thought process.
Editors have strengths and areas of mediocrity and blind spots just like writers. I would doubt the capability of an editor who does not speak well. But it is also true that the person who most needs an editor may be the least qualified to judge an editor’s ability.