There's no arrow on the 140 expansion tanks showing where the coolant level should be. The tank is black from the seam up; it's not transparent. The only way to determine if the level is correct is to open the cap and see where the coolant level is in relation to the tab in the corner. The proper procedure is explained in the owner's manual.

Here are some pictures of the 140 expansion tanks for those that are interested:

http://catalog.peachparts.com/ShopByVehicle.epc?q=1994-Mercedes--Benz-S4--0-Cooling--System&yearid=1994%40%401994&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6436%3AMBC%7C1532%3AED%7C10000069%40%40S420&catid=240909%40%40Cooling+System&subcatid=240916@@Expansion+Tank&mode=PA