What should you know about an editor's credentials?
First of all, you should know that there is no credential for editing that would compare with a teaching certificate, for example.
People come into editing from many different fields. Some are writers, some are teachers, some are librarians, and some begin in entry-level jobs in publishing houses. The list of starting points is long and varied. Most of us slide into the job while doing something else. One day editing takes over and we call ourselves editors.
Editing is a complex job.
When you hire an editor, you hire much more than technical expertise. The editor's skill in dealing with you as a writer and as a person are at least as important as her skill with your text.
And the truth is, anyone can announce, "I am an editor," and charge you money for what they do.
I think your real question is....
"How do I know if the editor can do this particular job?"
I belong to the Editor's Association of Canada. You might be interested to know that when I first created this page we were grappling with how to create a system that would rate an editor's credentials. That work is done. Because I did not want to work in a publishing house and the because the prerequisite to any of the other assessments was a test for working in a publishing house, I have chosen to forego the credential. I do, however, adhere to the ethics of the association, and my membership allows my clients to take me before the association if there is a dispute. This gives you additional peace of mind.
Here are some things to do when you are considering an editor: