Looking at my ZF and photos ...
I do not see how you can inspect without removing the bottom cover.
If you use a boroscope though the bottom drain plug, the boroscope would have to be manipulated around to the back side of the ring gear. That would be very tight.
I don't see how it can be done by removing one of the case side bolts on the left side drive covers - they are not through bolts.
HTH
B.G.