Checking rear end oil

3/8 ratchet, fits in the square hole on the filler plug
remove the plug and check for oil, it should be at the level of the filler hole
if not add until it starts to run out the hole

if you want to change the fluid, remove the rear cover first (dont forger to put a drain pan under it)
when all the oil has drained out have a good look at everything, gears etc.
clean the cover and mating surface, put a thin coat of gasket silicone on it and reinstall
then fill to the level/filler plug hole with new lube reinstal plug and your done