First, if possible, ask for referrals from writers you trust and whose work you consider high quality. Second, ask what style manual or manuals your potential editor works from and what they do if the information is difficult to interpret. (If US, it should be Chicago Manual of Style, 16th. UK is normally Fowler’s, 2nd). Third, ask them what dictionary they use. Compound words, in U.S. fiction, and the way noun phrases are hyphenated or not is decided by dictionary use (Merriam-Webster’s). Fourth, ask for a sample of their work. When he or she suggests changes, *ask why.* Authors often assume a person knows what he or she is doing, and when it comes to grammatical and style issues, they often don’t ask them to back up what they say with outside information. I work from Chicago Manual of Style, 16th Edition as well as Garner’s, 3rd, and my clients will all tell you that at some point, I quote from these books with reference and/or page numbers . . . well, except for the one I have who knows that I know what I’m doing and just wants me to tell her what to do. Ask if they belong to any editorial associations or groups, online or off. Finally, ask them to explain compound predicates, noun phrases, and restrictive clauses. Ask if mass nouns (a.k.a. noncount nouns) take a singular or plural verb. LOL to self: just ask what a conjunction is. Ask the difference between further and farther, already and all ready, its and it's. Some of these are simple things that some writers know. However, non-editors won’t be able to answer the more complex questions because they have never bothered to ask and answer them for their clients. Any complex grammatical or usage question will throw a non-editor. (If they give what seem to be bogus answers, feel free to e-mail me.) If they don’t know this stuff, run!!! He or she is a pretender. Expect that a true copy editor will charge more,–it takes longer to do it when you know how to do it right–ask more questions of you, and make you work your a$$ off. Some copy editors will also offer proofreading services, and what that includes can vary. If you want a beta reader, don’t pretend you’re working with an editor, but if you’re receiving high-quality advice, I see nothing wrong with paying for a beta reader. But I would only pay about $25 for it, at most. Copy editing is substantial work, and proofing is a bit more intensive as well. Reading and giving an opinion isn’t.