All editors will offer you a sample edit when you are deciding whether to team up with them for the editing process. When you're investing in editing you want to be sure that you are getting what you want. Here is a checklist of three things that you can learn about your editor through your sample edit.
The number one thing a sample edit for is to demonstrate the editor's skill.
You should already have some idea of this from their portfolio, testimonials, and credentials, but a sample edit will directly relate the editor's talents to your work. If an editor turns back a three-page sample that is very scant of comments then it is likely that they aren't all they're cracked up to be. Equally, if the comments are vague and unhelpful they likely don't have the expertise to give you appropriate advice.
Following on from that, the edits should be constructive.
Great. You get your sample back and it is full of edits. But they're overwhelming, poorly organised, and at times harsh. This isn't constructive. Sometimes a small scene may be able to attract a seemingly disproportionate amount of notes, especially when it is an action-filled scene. However, a good editor should be able to organise these notes so as not to overwhelm you.
More importantly, any criticism should be polite and helpful. Self-editors may find it quick and useful to mark up sentences as "terrible, change it", but that is totally inappropriate from a professional editor. Criticism from an editor should use proper terminology to explain what they feel isn't working well with the scene/sentence and offer suggestions for how to amend the sentence. This note should be phrased as an opinion rather than a fact. Ultimately, the author gets the final say over what edits they implement and what they ignore.
Your sample edit will show your editors professionalism
Beyond correctness of language and skill with developmental edits, a sample edit will show other traits that are important to consider when hiring an editor. The editor should provide a timeframe for returning the edit to you. They should meet this timeframe. Emails should be professional but friendly (see my blog post Is it important to like my editor?). They shouldn't be pushy or entitled to your custom.
When you choose an editor keep in mind all of these things and make sure to ask lots of questions. Be sure to also check their portfolio, reviews and credentials.