When the engine is cold there should be coolant visible when looking down into the tank thru the cap opening. If you have trouble seeing the level, pour a little water (30ml or so) in and watch.

DO NOT fill to the opening as the upper black portion of the tank is for expansion when the system is hot. Drinking water is okay if not too salty. If so, mix with distilled water or use distilled water instead.

You might want to have the coolant concentration checked or buy a simple tester. MB recommends that the coolant system be drained and refilled every 3 years.