I do not know if this can be done on a Mercedes but on the other cars that I have been working it is done as follows: The valve cover is removed and the engine starts and idles (with the valve cover still removed). Then, get a .020 inch feeler gage and slip it between each valve tip and rocker arm. When you get to the one with the bad lifter the noise should change markedly.
Now I am not sure if it can be done in a Mercedes but all the cars I have been working on (primarily domestic V8) that is how we do it. Hits the target every time.