We have such a magnet for the dishwasher at work. Problem is that people are unreliable at actually flipping it. I just wish there were a display that would say what the current dishwasher state is.

If they don't already exist, there's got to be a market for dishwashers with a built-in clean/dirty indicator. With smart appliances nowadays, it doesn't even have to be a physical display (in the home.)

It's hard for the machine to actually know if it has been unloaded...

So many people in an office open the clean machine, remove one mug, and then close it again.

Should the machine now show dirty or clean?

It should clearly take a photo and blare the warning siren.

The weight of the drawers is a pretty easy indicator for of it was emptied since last wash. Until it hits empty+10% it's still clean.

At some stage it will be completely empty[1], so it should be possible to weigh the contents.

[1] No readonable person could possibly consider begin to fill it before it’s completely empty, right?

You get to a state where there’s one mug in the dishwasher. Can you tell if that’s clear or dirty without closet inspecting the mug (and even then depends on your definition of clean. Imagine you filled the mug with pee, then emptied it, then put it in the dishwasher. Could you be certain of knowing the state of that mug alone?

The heat of the items gives a clue, as does the number of clean looking items, but there are ways to break that system.

I have literally never once worried about whether my family are secretly peeing into mugs, emptying them and planting them into an empty dishwasher.

I guess now I will.

If you are weighing it just consider it dirty once the weight goes up. This will say clean even when empty but that seems not incorrect.

I think the only problem would be if someone removes the last clean item and adds a heavier dirty item at the same time.

My flatmate in university: "should I unload the machine? Nah I will just put my bowl dirty with yogurt in, to drip it onto the clean plates"

How would a machine even detect such situation?

Some machines, even very cheap ones, have an indicator light for the cycle finished. Some just show if the cycle is finished and so it indicates if someone has opened the door since then, but better ones keep that indicator lit until the next time the door is fully closed giving more control over the light.

Yeah, those magnets only work with people that read them. We also bought one for the cat because nobody knows who fed the cat, if anyone. The cat looks fat and every now and then someone gets a dirty bowl. The solution for the dishwasher is simple. I fire alarm in the dishwasher that goes off when this 2 events happen: 1. The door was open after a complete wash cycle. 2. 5 minutes have elapsed from the door opening and there still dishes inside.