I had exactly the same question as I'm busy formatting an ebook that is to be published on Smashwords and it's the first book I've done so I wanted to see that my formatting was working. I emailed Smashwords and received a reply saying that I should just upload the book to them and then immediately unpublish it. This would let me download the converted files without anyone being able to see the book.
Well I've done this but found some formatting errors that I'm not sure how to fix (bullet lists behaving strangely, numbered lists indenting strangely, etc). I'm obviously going to have to experiment to find out how to fix these problems, which will result in multiple publishing / unpublishing actions. I'm surprised that Smashwords don't already let you upload and check the results without actually publishing the ebook, as I'm sure most people would want to check formatting.
Oh, and it's a cookbook that I'm working on, hence the bullet and number lists.