When the engine is OFF, open the hood and look for the belts on your engine. Since there are several, you'll want to test the belt tension.
A non-scientific way to do it is to reach down and where the belt is mid-way between two pulleys, push down on it. Mercedes take a firm belt tension so therefore the deflection (how far it can be pushed down) should be less than 1/2 inch.
You can check the tension also at the alternator, by using its fins and try and turn the alternator. This done best when the car is completely cold, such as first thing in the morning. The alternator is on the lower right of the engine, or lower left if you are facing the engine.
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