Generally, the simplest way to get a sense of what a —so far unknown— proofreader or editor offers in terms of being able to improve a manuscript is to find a short, sufficiently representative excerpt of the manuscript, and edit it. This shows what kind of things an editor pays attention to, how they communicate issues with the text, and provides a basis for the client and editor to discuss what kind of edit is best.
With nearly all of my clients, therefore, I've given a sample edit before agreeing on the terms of the edit.
Some points:
I use abbreviations for my comments - these are explained in my guide to editing abbreviations;
There are some ways in which my sample edits will typically be unrepresentative of the editing I would later do in a line edit on the full document. Most importantly:
Far fewer edits in the full edit will have comments: when I make the sample edit, I will have made only a cursory investigation of the manuscript, and I will not usually have had a chance to ask questions about the text. By the time of the line edit, I will know much more that helps me resolve these issues for myself;
One of my aims in the sample edit is to expose as many issues as possible. Typically, therefore, I will choose a sample to edit that is particularly problematic, or that illustrates things that should be discussed before editing begins.
