I'm guessing you are new to the tractor. There is a window for hydraulic fluid in the rear axle housing. It's on the left side, viewed from behind the tractor. If you see any fluid in there at all, it's OK. Ideal is in the middle.
If this is a new tractor to you, when you change fluid, also change the front axle fluid. It's separate from the transmission/hydraulic fluid. Drain each side and the fill is on the front axle housing. You can use either 80w-90, UDT or SUDT. I have SUDT in mine.
The Kubota has only 2 reservouirs. The back is the transmission, hyraulics and axle all itogether. The front is the axle.
It's a good machine - enjoy!
jb