There is a fill indicator in the form of a small plastic tab. When standing at the side of the car, look down into the tank opening. You should see a 3/8"x3/8" square at the 9 o'clock position. It is approx. where the top (black) and bottom (clear) halves of the tank meet. If it is missing, just use the dividing line as a guide. It is an expansion tank so level isn't critical. Check when cold.

If the coolant level is low, it should be easy to see. If not, maybe that's not the problem.

Use MB anti-freeze diluted to 50:50 with distilled water for best results. OK to add water if it is only a small amount (< 1 liter). Coolant should be changed every 3 yr regardless of mileage.

Investigate loss of coolant.