A small, upright glass makes for a great indicator.
When the glass is full of water it means that the dishes are clean.
Empty the glass after you put the dishes away: An empty glass means dishes are dirty.
As long as you remember to pour the glass out, it will work every time.
No clutter, no mess, and you barely have to pop pop the dishwasher to check.
How to Hack the Kitchen Tools You Need (+ Why Not to Buy Them)| Food52