I actually had a da*n good question to use at the end of an interview. You usually will get a chance (If you don't then that should start the alarm bells going)
So the one I use is "Do you have any concerns with me as a candidate for this job ?" This is great because it allows you one last chance to combat any negative thoughts and more importantly, I found, allows you to see how your prospective employer reacts. This might mean that you think twice before accepting an offer from them. I know that in one job I went for the interviewer seemed quite insulted that I'd asked the question!
Scarlett.
BTW, Just remembered, it puts them on the spot and makes em squirm and after what they just put YOU through for that last 30 mins/hour, it's only fair