You definitely need to check the belt.
If you have an operators manual, you need to read over it carefully and highlight important service intervals.
If you don’t have one, you can download one from Exmark. You will need the model and serial numbers.
I’m not sure how your particular mower is set up.
The inspection port is located on the inside center of each hydro. It should be about 2” from the top of the casing. Some manufacturers use these ports to connect to the expansion tanks.
There is also a plug on top of the casing. Some manufacturers use this port as a vent. If you have expansion tanks, open these top plugs to allow you to fill it faster. If you don’t have expansion tanks, open the inspection ports and fill through the top plugs. The fluid will begin to run out of the inspection ports when full. The procedure is listed in the service manual that I linked to.