A great closing question that you can ask for which you want them to tell you "no" is if they have any other questions or anything else to add before you finalize your proposal and prepare it for their approval. A similar one is asking if there is anything else they want to discuss before you prepare to get started with their project.
There may be additional issues that they want to cover, and there may not. Either way, you are that much closer to a final solution and approved agreement.
When structured the right way, questions can be asked that will yield very helpful "no" responses.