Or if you have an already incomplete set of scrabble tiles: attach magnets to the backs of the ones that spell "dirty" and "clean". Whichever isn't scrambled on the door is the state of the dishwasher.

Or simply don't rinse the dishes before you put them in[0]. I've never had trouble telling.

[0] Exceptions: uncooked eggs, yogurt, and for some reason, salsa? None of which ever come off for me if they sit for long before you run the dishwasher.