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.

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.

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