The first thing a ghostwriter should do for a prospective client is to evaluate his or her idea, rough draft, notes, etc. Many ghostwriters (and virtually all ghostwriting companies) will either skip this process entirely or tell you what you want to hear because they want the job.
Before there is any contract negotiation or talk of writing/editing your book, a good ghostwriter will give you an honest assessment of your proposed work. Is it marketable? Does it fit an existing genre? What is the proposed length, and is the length sufficient for submission? (Novels, for example, are usually not considered unless they are at least 65,000 words in length.) Is the market already saturated with your concept? If you have a rough draft, does it need major editing and revision or simple line-edits? These are all questions that need to be answered before work begins.
Always insist upon a detailed assessment of your project.