Once you start searching for an editor and start receiving responses to an advertisement you may have posted, you will naturally want to find out if the editor’s skills are as strong as his or her résumé claims, and if he or she is the right editor for your manuscript before you hire him or her.
All major publishers give their potential freelancers a copy test and a proofreading test to see if they are qualified to work on the books they publish. Private clients usually send a sample of the manuscript to be edited by the many editors who respond to their ads to see if their skills match their needs. I do not see anything necessarily wrong with this, but clients need to think clearly about what part and how much of the manuscript they are going to send as a sample for editing.
Whether it is fiction or nonfiction, it is always best to send the opening pages of the book to the editor, that way he or she can get a taste for the subject and your writing style. Once editors review the sample, they themselves might not feel comfortable with editing the manuscript because the subject area (especially for nonfiction) may not be in their area of expertise or interest. It is often easier and more enjoyable for an editor to work on books whose subjects he or she is familiar with or has a strong knowledgebase in, which would then make your book that much of a better read as a result. So, give the potential editor a sample from the beginning of your manuscript so they know what they will be getting into if you offer them the job.
Next we come to the length of the sample you send out. The sample should be long enough so that there is something to be edited and the editor can display their skills, but not too long that it would take longer than twenty to thirty minutes to review. I cannot tell you how many times I have received samples from potential clients that have been over five pages. This is definitely not a good idea. The length of your sample should never exceed five pages because you are asking the editor to take time out of their usually busy schedule to edit text for free, and an editor’s time is worth money, and most of their time is spent working, as I’ve mentioned previously, on manuscripts that pay hourly and are often on tight deadlines.
So, my recommendation for when you send a sample to a potential editor is that it be from the beginning of your manuscript and no more than three to five pages. Your future editor and all the other potential candidates will appreciate this, as it shows you have an understanding of the editing process and the time and energy it entails.