Hydraulic reservoir/tank is the whole top cast iron part from the main transmission housing on up. Kind of a process to get it off. However, you may be able to see how dirty it is by removing just the top cover off, where the operator's seat would attach. If it has a cab, it's going to be quite the chore.
The video above is of a Category II 3-point on an 1855 with the fenders off. Category III will be a little different but not by much.