The only way I think this can work is if the customer who reports the incident describes exactly what they see in the report. Then the human who receives the dirty car and cleans it verifies that the car is not only dirty but meets the description of the reporter. How else could "dirty car" reports be audited?
In another case, a customer could actually throw garbage in the car and then report the car dirty - causing the previous customer to be fined. The only safety net against this I know is to record video inside the car while a customer is present.