When the original Advisory Board wrote the inspection standards for the state, they included a special provision for air conditioners. The standard specifies that the home inspector working in Washington State test the temperature differential on the air conditioner.
Temperature differential is just a fancy way of saying that we measure the temperature of the air going into the air conditioner - say it is 80 degrees - and measure it as it comes out of the air conditioner - 61 degrees. We do rudimentary math and arrive at a 19 degree difference. The range that I use (and most inspectors are close to these numbers) is 14 to 24 degrees of difference. Too little cooling and we have a problem. Too much cooling is also a problem, though, as this can indicate poor air flow and a host of other issues with the cooling plant. For the actual diagnosis of the system, I punt it to the experts.
The exception is when the outside temperature drops. The condenser unit for the system (that’s the part outside) gets too cold, the oil in it gets ‘thick’ - that is, the viscosity, its ability to flow, is low. Trying to move that cold oil through the system can damage it. Thus, when the outside temperature is below 60 degrees, the standard allows us to note that fact and not test the air conditioner for operation. That does not mean it is not inspected - we’re still required to examine the readily accessible components and report any deficiencies that we see.