As an author, I learnt extremely quickly that there are predators in the forest of books. While legitimate author services can help you present your books and titles in their best possible light, unfortunately, there is just as many that would love nothing more than to remove your hard earned money from your pocket and into theirs.
1) What are your qualifications?
While it's okay to not have a degree in English, or a degree at all, most professional editors DO. Failing that they either are a serious English Lit nerd or have something slightly below that (ie: Journalism at the College level)
2) What do you like to read when you're not working?
... And the answer should not be, "I don't read anything unless I'm paid to edit it." If it is, run. Generally, and the only reason to ask this is to determine if they're a good fit, not to find out if they're qualified. If the books they *like* to read run the same as yours, or, at least, the same style, then you know they will probably "get you", and more importantly, like and understand your style of writing.
3) Ask for references, and if you can contact those references.
For the same reason an employer asks for your references... only now you're the employer and that editor your employee for the purpose of your work.
4) What are your rates?
Be wary of a price that is too low, or even one that is too high. Shop around and find out what most freelancers are offering, and even take a look into the guild. Yes, there is a guild for freelance editors, but be warned the suggested prices will be staggeringly high for an indie. However, if the editor you are considering is part of the guild you can rest assured that they likely are a bonafide qualified editor with references to boot. A good spot to start, even if you're not from Canada, is here:
Also, check out this site, which is a creative service trading group. It's like bartering but for authors and anyone else in indie publishing: