Boeings (737, at least) have an "ALT DISAGREE" indication if you are still in QNH if you pass the transition level on climb out. The reverse is true on descent.
I'm not sure which 737 you fly ImbracableCrunk but on the one I fly the ALT DISAGREE message only appears when there is a difference of more than 200 feet between captain's and f/o's altimeters.
If you pass transition altitude without setting STD then the QNH or QFE setting will turn amber and become boxed, and likewise on descent the STD will turn amber and become boxed if you forget to change at transition level.