Don't take the axle tube bolts loose, some have a free nut on the inside and you can't tighten it back up!
The proper level is to the bottom bolt on the round inspection covers under the seat.
I would not pull the shifter cover just to look, that is asking for trouble! Instead take the right round cover off. You can also take the left cover off, but it has to go back on first so you can get the PTO linkage back in place.
Those are known for collecting water in the cases. It gets in by condensation and around a bad shifter boot. You can loosen one of the drain plugs and see if water drips out. If there is water, it will be on the bottom. If the oil is milky, it has water emulsified in it and needs changing.
If you change the oil, refill it with 90w mineral oil, or 15w40 engine oil. It will hold 4 1/2 gallons. If you want, you can flush it with diesel. Don't be tempted to remove the big square cover on the bottom! That is not a cover, it is the hydraulic pump. Loosen it and you will start a leak! It won't come down without further disassembly.
When refilling the oil through the plug on top of the trans, fill it very slowly. The oil had to make it's way to the rear housings, and the passages are small. Fill it too fast and it will dump in the clutch! Yes, all the housing are interconnected. You can fill it through the inspection covers if you like.