I believe the only bikes that come with stock peepholes for checking gear oil levels are the Kawasaki's. I don't know how accurate they really are, but personally I don't think checking gear oil is even a big deal. It should really only be in there for a month at most during riding season. Unless you notice severely rich engine conditions that could point to a leaking right side crankseal, as long as you filled it up with the correct amount you should be fine. There is tremendous debate over what kind of gear oil to use. Many say 80W Gear Saver stuff from Bel-Ray, others suggest ATF fluid, some suggest 10W-40 or 10W-30 conventional oils. Personally I used Mobil1 Redcap 15W-50. But it's my belief that as long as you change it frequently, and it's the correct amount any oil is fine.