If you have a GM A6 compressor which was common on a lot of MB's at the time, There should be a cap on the bottom of the compressor reservoir. Run the AC turn it off and crack the cap. If vapor only comes out you need more oil. If a bubbly foam of oil comes out your good. That was the proceedure for GM vehicals that used this compressor at the time. MB may have a different proceedure.

John Roncallo