Why the eye?
Surprisingly, very few people have it in them to be good proofreaders regardless of their knowledge of the English language or their ability as writers in their own right. This is because it is human nature to read something that is flawed, make unconscious corrections in our minds, and not be able to pinpoint the problem. We may get the feeling that a passage isn't polished, but, for the life of us, we can't say why. This is the case for almost all of humanity.
Morgan is the consummate proofreader. She proofreads bus stop benches, billboards, food ingredient labels, and movie credits. She couldn't turn off her proofreading if she tried. Birth defect? Perhaps. But this is the trait that makes her fast, accurate, and thorough. It's what places her in the tiny group of people that are truly good at what Morgan does.
When good writing is expressed in flawless English, the reading experience becomes transcendent. Images and experiences occur effortlessly in the mind of the reader. The thrill of reading is amplified as the audience loses awareness of the written medium. This is what your readers want, and this is what you want for your readers.
How to choose a proofing editor
This is probably easier than you'd expect. Google a handful of proofreaders' websites. Then look at each one carefully. Amazingly, more than half of them contain usage problems, grammar faults, and typos. You may not consider yourself a copy editor, but as an author you love words and written communication. You can spot some of the problems if you try.
These imperfect websites belong to individuals who mean well, but who don't have the gift. Sloppiness in their writing will almost certainly foreshadow sloppiness in the work they do for you.
Next, look at each of the surviving websites and try to get a feel for the personality of each editor. Trust your instincts. This will allow you to eliminate prospects with whom you might not enjoy working.
Now, choose two or three individuals who pass the first two tests, and see if they offer the chance to sample their editing service at a very nominal cost. Most do, and all should. Then, choose a sample of your work and send the same selection to each of the candidates. Feel free to include intentional flaws if you are so inclined. Expect a prompt turnaround. It takes great energy to do what you do. You can't afford to lose your momentum on a project.
When you have the proofed works back, examine them side by side. Notice the errors that were missed by each of the proofreaders, and bear in mind that perfection is attainable. Look at the writing you submitted and the suggestions made, and trust your gut.
That's it. You've done your due diligence, and you are ready to roll. Our best wishes to you in your endeavors.