Which of the following is NOT a method to verify a system's refrigerant charge?

Measuring the ambient air temperature is not a direct method for verifying a system's refrigerant charge. While outdoor temperature can influence the efficiency and performance of a refrigeration system, it does not provide specific information about the refrigerant charge itself. The other methods listed are all practical approaches for assessing whether a system is properly charged with refrigerant.

Checking the temperature of the evaporator coil can indicate how effectively heat is being absorbed, which is closely tied to the charge level. Observing compressor activity helps determine if the system is operating normally, as compressor behavior can be affected by refrigerant levels. Analyzing pressure gauges against standard values gives a clear indication of whether the refrigerant charge is within the recommended limits, making it essential for troubleshooting. Therefore, while ambient air temperature has its relevance in the context of system performance, it does not serve as a reliable metric for verifying the refrigerant charge specifically.