Spotting dirty air conditioning coils or mold during a home inspection can be easy. I make it a habit to open up the vent on the cold air return under every air handler. About 30% of time I can see the open coils inside the unit. I commonly see water damage of the return compartment, mold, fungus, rust or dirty coils. It’s hard to describe the visible difference between mold and dirt. I can try by saying that mold has more of a live look. In either case, this is restricting the flow of air through the furnace and causing stress on the fan motor, reducing efficiency and cooling. It needs to be serviced by an HVAC technician. This is usually caused by the lack of a filter on the return vent. It is recommended to change the filter every month but I think that is a little excessive. I’m a busy guy and end up doing it every three months, which is a little too long between new filters. Do as I say, not as I do.