I rinse my dishes off before putting them in the dishwasher. If the dishes have been in there long enough to dry (e.g. overnight), they can visually appear to be unsoiled.

Enough people have this concern that there is a market for magnets and such to put on the dishwasher to indicate whether they are clean or dirty.

Do you live alone? If more than one person runs the dishwasher in your house, then you have to go off of how the dishes look. Many dishes don’t look visibly dirty after using them. You can root around in there, looking at them all, and hope that you make the right judgement call (otherwise you’re putting away dirty dishes or adding a dirty dish to the clean dishes) or you can just glance at the detergent reservoir and know that closed=dirty and open=clean. It takes no more time than any other way of doing the dishes because you still have to fill up the detergent at some point.