You can Buy some Block check at the Local auto parts store that will test the coolant for Blown Headgaskit/Cracked head,block
Inspect the The Radiator cap seal and Spring If the seal is Shot/Dry or the Spring binds Replace the cap..It helps to pressure check the cap..But since you dont have a Cooling system pressure Pump ..I Would Just get a New MB radiator cap
The Cooling Syetem Should NOT be Down any amount after that drive Unless You have a Leak
You Should Try to see if you can Borrow a Coolant system Testor
many Auto part stores will Loan you one To test The System ..To help you find Leak.