What are some of the questions one might ask a prospective supervisor? This is in assessing their interest as supervisors and how they can contribute towards steering you towards that ultimate goal, that of completing one's PhD.

It's kind of hard really, because even if you ask them they might not be honest, or they might have the best intentions but can't or won't fulfil them. In some ways, you are better off talking to previous students of theirs if possible because that might give you a better idea. Be sure not to confuse their outward persona with their potential supervisory style, as these can be vastly different.