Please show us when we lose an accepted answer
Why doesn't the reputation graph show the rep lost when someone removes the accepted answer to a question I've previously answered? Where else can I find this information?
As of 2011-09-22 you can see in your reputation report if an answer was unaccepted.
Original answer below the line.
You can't. There isn't any feasible way.
You'd have to simply notice the absence of rep where you had it before, either on the reputation graph or on the recent rep page.