I'd love to say I track these hiccups by some orderly process (I'm old enough to remember the ancient days of having to start and restart System 5 or so to figure out which extensions worked or didn't). But in this case I eyeballed them and this was one of the usual suspects, although not in the context of shutting down. It was some other problem. So it's kind of in the back of my head every time I troubleshoot. Utilities like Kext Viewr and Kext Wizard can help sort out such problem children.