And, to paraphrase another poster, be a honey badger about it. "And then what happened? And then? Why did you do it this way? Why did that fail? Why did <cause of failure> happen?" until you get to the bottom of the story or hit a wall ("dunno. My boss took care of that"). If the candidate can explain what the root cause of the problem was, how they identified that root cause and then proceeded to fix it, that gives you a pretty good idea of how tenacious they are.