You can't know all of these things upfront, but they're good things to watch for! Their absence should raise red flags.
They have a money-back policy. This seems like a no-brainer, but on the off chance that you get halfway into an edit and decide they're really not for you, you're going to want to know what they're money-back policy is. What their policy actually says is less important than them having one, but I'd say a 50% refund on half-finished-then-canceled work is pretty standard.
They aren't going to charge you thousands of dollars. Even a 50% refund won't get you very far if they're charging you two thousand dollars, pounds, or what have you. I do know well-established, respected, loved freelance editors who charge this kind of money. (And that's fine--I should say don't pay this much if you've never heard of them.) But you can find cheaper without sacrificing quality.
They come off as a generally positive and encouraging person. Obviously, it can be hard to know these things through email communication and whatnot. But sometimes it pays (or in this case, saves you money) to go with your gut. Just because you don't click with them doesn't mean they're a bad editor, but it does mean they're a bad editor for you.
They finish according to the timeline and preferences discussed. As someone presenting themselves as a professional, they should act like it and deliver on the promises they made. Now, life does happen, of course. Even professionals can't remember everything or keep relatives from dying. However, they should still try their best and inform you.
Which brings us to ...
They're upfront about your preferences and their services. This is another no-brainer, but it's surprisingly easy to discuss things, hire someone, and then realize how much you should have talked about after the fact. Do they ask what your preferences are for things like the Oxford comma, or do they force their opinion on which way is correct? Do they list the type of edits they offer or have any testimonials on their website?
They reassure you that the story is ultimately yours. Okay, this one isn't a must, but it's a very good sign. It goes hand-in-hand with not forcing the "rights and wrongs" of grammar on you. Many things are personal preference, and they should be there to inform you of the rules and their preferences, but ulitmately let you make the calls! (Hint: you make the calls whether or not they're cool with that--they can't stop you. But they shouldn't try to stop you either.)
They communicate during the process. There isn't always a lot to say during a project. But if they have a two-week vacation in the middle of editing for you, they should let you know. If they have an issue with some of the content, they should let you know before a two-star review. If they get confused, they should shoot you an email or at least leave you a comment in the document. Communication is a key to keeping you--the writer--at the heart of the service.
Anything you'd add to this list? How do you find reliable editors?
It's been a while! And I've mostly been working on Martin Legacy edits with my editor and editing for my own clients. Thus, this post. :P What's new?