Hang an implement on it and raise the lever. Look through the cover on the side with a flashlight, specifically towards the cylinder, and up towards the front of the cover. Pull the cover opposite the PTO shifter, the oil has to be going somewhere. On this tractor, the filter can be pulled from the shifter side without removing the lift cover. Should be a wire mesh that you can clean. It'll look like a barrel standing at the rear of the pump, some don't have one at all.
You can also check the transfer tube o-rings by removing the two bolts that hold the T block on top of the lift cover. Do this with the arms down. On some the tube is welded into the block, others the tube is liable to remain in the pump when you pull the block off. In which case, you pull it up out of the case, check the o-rings on both ends (there's backup rings, as well) and replace as needed.
Most often, I've replaced the control valve on these. The spool oscillates and wears where it rides in the washers. Pressure is low and the links bleed off. You'd have to pull the pump and remove the spool to verify, but it's fairly simple once you have the lift cover off. There's a pin with two nuts on each side, and you have to pull the PTO shaft.
If you haven't pulled the lift cover yet, remember to pull the rolling pin out of the control valve linkage through the side cover first.