You can dye the oil, red with... MT3668: Hydraulic Oil Dye Fluid (John Deere, google it)
Kubota did make a dipstick (no longer available as far as I know) that were to replace the sight glass on the sides of the transmission when customers complained that the 2004 to early 2006 "standard" models were too noisy (sounds coming from the overriding, racthing CAMS). Which occurred when the PTO was on but the engine speed had suddenly been slowed down (the momentum of the mow blades were spinning faster than the PTO was) the CAMS to power the PTO were designed to skip over so that power could not be forced back into the engine via. PTO...
Kubota made that change to raise up the oil level (~1.8 gallons) to put the CAMS under oil level.
I think later models of the "standard" models might have switched to dip sticks... on the cross over metal section...
EDIT: The next model up has a Dip Stick but Kubota changed to transmission case for that to be done. L3901 case vs. L3200