1- they will check with a Star diagnostic scanner , Mercedes can read the oil level off the oil level sensor and the hours and mileage since last service .
2- check it thru the dips stick, they have the dipstick to insert to check
3- or to replace they use a suction machine to change oil like below
AP18.00-P-0101-02Z Suction Off Engine Oil ENGINE 102, 103, 104, 110, 111, 112, 113, 115, 119, 120, 133, 137, 157, 166, 270, 271, 272, 273, 274, 275 (except 275.981/983), 276, 278, 279, 285, 601, 602, 603, 604, 605, 606, 611, 612, 613, 616, 617, 628, 642, 646, 647, 648, 668 up to 8/31/00
Engine oil should only be extracted when at operating temperature. Extraction of the engine oil can be commenced immediately after the engine has been stopped. It must however be ensured that the extraction procedure is not stopped until at least 5 min after the engine has been switched off and the old oil filter has been removed. This gives the engine oil time to flow back into the oil pan. If necessary, the extraction procedure must be started several times before the 5 minutes have expired. It will otherwise not be possible to extract all of the engine oil. Any quantity of oil remaining in the engine can result in overfilling after pouring in the specified oil change quantity. This can then result in damage to the engine and catalytic converter. Engine oil should only be extracted when at operating temperature. 1 Pull out oil dipstick and extract engine oil through the guide tube. Engine oil suction unit - mobile gotis://S_18.1_02.0
Please let me know if that work for you
ASE Certified Technician