1. Choose a specialist academic manuscript editor rather than a generalist editor.
2. Know exactly who you are hiring. It is important to know exactly who is going to be working on your project, and to be aware of his/her qualifications/experience. I recommend that you hire a freelance editor (not a large editing company) and ask him/her to confirm that he/she does not subcontract projects to third parties.
3. Choose an editor from an established listing of editors. There are plenty of listings online; here is one example: The Editorial Freelancers Association. Of course, there are never any guarantees, however, I believe that it is safer to choose an academic manuscript editor from a bona fide directory, as opposed to a business that is not listed in any official register.
Stay tuned! I will be doing more posts on this topic in the future.