I like the idea of embedding “fine print” into the initial email. I also think it’s a good idea for an author to check references; the questions Val suggested are great. Third, as another commenter said, an editor can give the author a general assessment of their book, which will help the author decide if that particular editor will be a good fit or not.
It’s always sensible to follow due diligence in submitting your first draft of your book to an editor by making as fool-proof a copy as possible for you because first, it is your own creation, and secondly, a horrid draft sent your editor can end up frustrating both you and him or her. Make your own corrections to the extent you can, so that will make the job of your editor that much lighter and the end result would be a lot more effective.