Instead of running scripts ever .1 seconds to check this I would prefer an ON Event block. But reading the events list nothing jumps out at me.

http://www.creationkit.com/OnTrackedStatsEvent_-_Form, probably.

Maybe OnTrackedStatsEvent() (http://www.creationkit.com/OnTrackedStatsEvent_-_Form) could help you to find an indirect way. It tracks changes in the misc stats of the player http://www.creationkit.com/GetPCMiscStat . It need to first use the RegisterForTrackedStatsEvent() command and there is not a 'single' version, so it could be prone to bloating depending on how you use it.

I see JustinOher has been quicker