How to find master slides being used?

I recently found a solution to the impossible-to-find '1 slide' that used a particular Slide Master, even after going through every slide. A Draw from Question Bank uses the 'Blank' slide layout from whatever the first / top Slide Master is. It can't be changed, or even seen, at the slide level because the draw is not itself a slide. The solution is to delete the draw, then delete the now-orphaned Slide Master, and re-create the draw (which will now use the 'current' or top Master Slide blank layout. Hope this helps!