Based on my very limited experience:
Look for oil leaking onto the shelf below the bell housing, if it's engine oil, the rear main seal or oil pump cover gasket is leaking. Might consider replacing both if you're in that deep. If it's hypoid oil, the gear box input seal is leaking.
If you determine that the RMS or oil cover is leaking and must be replaced, BLOCK THE CRANKSHAFT before you remove the flywheel. Some folks have trouble getting the oil cover screws out. An impact driver helps. Northwoods Airheads sells allen head screws to make this a non-issue going forward. You'll also have access to the clutch plate and spring, which you can measure for wear.
If you remove the swingarm to completely remove the gearbox, you'll have the chance to inspect the final drive splines. Use new bolts (short, without washers) when reattaching the driveshaft to the gearbox output.
I'm sure there're are things I missed. But I just went through this on my /6, and these were my learnings.