I have heard that you can request readers' comments from publishers after you have submitted a manuscript to them. What is the best way to do this? In the query letter? A note after you have been rejected?
You can request them. You may or may not get them. Depends on the company policy. Some companies don't mind sharing the comments; others won't.
I would make the request in the query letter. Some publishers file readers' comments and keep them for a long time. Others simply note them in their log and toss the originals, in which case, by the time you get your rejection and request to see them, they may be long gone.
FYI--Readers' comments refer to the practice of editors/publishers sending pages out to trusted readers with a comment form. If all the comments are favorable, chances are you'll be accepted. If they're not, you'll be rejected.
However, many submissions are rejected before they go out to readers. Readers are only involved after the editor and a few in-house employees give the manuscript a thumbs up.
Tangent question: What if the editors like it but the readers don't, or vice versa? Who decides? The marketing department.