No pressur on gauge. How do you check an oil pomp of a MB 300D?

Have to remove oil gauge sending unit and check pressure with a master gauge. Sender or gauge could be faulty. I would not suspect the pump unless you see signs. First of all, does the gauge have pressure when the oil is cold? If it does and the pressure drops off when the oil gets hot, then you cold have a pump or relief valve problem. Removing the pump and inspecting should be your last resort.