Oil level control question

I just read a thread in another forum that found interesting because it deals with a situation that I have asked myself many times.
The rack operation manual warns the technicians that the oil level in the control´s sightglass is not always a true indication of the actual level in the crankcase. If this is true, how can someone check the true level in the crankcase, if the spot where the compressor sightglass is suppose to be installed is occupied by the oil level control? Additionally, I believe that the original compressor sightglass plate has been removed and fitted to the oil level control.

The sight glass installed on the oil level control will always show 1/16 to 1/8 level of oil even if there is no oil. this is caused by oil lying on the crossover tube. never trust the sight glass if it is that low. Use a tortch to heat the side of the compressor like checking fluid level in a recever tank, where temp raises you are above the fluid level where the temp remains unchange you are below fluid level. This is not as accurate with the compressor walls being thicker than a recever tank.