the ideal temperature of the evaporator coil (the part that the air passes over to cool the air, or technically remove humidity from the air making it feel cooler) is just above freezing. therefore if the ambient temperature is below the temp of a functioning evaporator coil, you won't know if the a/c is doing it's job or not. the only way I can think of is to put the car into a heated garage or warehouse long enough for the temp in the car to get warm, and then check a/c operation.