Here is an easy method in determining your cooling efficiency:

With your A/C system on

Take the temperature of your high pressure side condensor inlet hose and subtract it from the condensor outlet hose temperature. This will give you the temperature rating of efficiency of the cooling capacity for your system.

 

My local A/C shop sticks a thermometer in one of the vents inside with the A/C on full blast.
He told me how much colder than ambient it should be, but I can't remember. LOL
I think he said 20 degrees, but it may have been 40, but 40 seems like too much of a delta.

 

Yeah that method will vary, based on your current ambiant temperature. If you check at the condensor, best case scenario would be perfect air flow to change the state.