Most of the transmission is super durable. As I recall there's one needle bearing and the rest of the gears run on plain (sleeve) bearings. Just disassemble the shafts one at a time, take a picture of the aseembled shaft and lay all the parts in order as you take them off. Check the gears for sloppy fit on the shaft, they should turn very freely but not "rattle around". The gears engage with "dogs" on the sides, check the dogs carfully for any problems, they should not be rounded off: rounded ones can be undercut by an experienced person, any missing (broken off) dogs means new parts are required. The actual gear teeth rarely break or get damaged, if a dog has broken off it can get into the gear teeth and really mess them up though.
The shift drum grooves and the rollers that run in them can wear, giving poor shifting and false neutrals.
The shift forks wear at the tips and can bend, again poor shifting and false neutrals. The forks may be identical, so you can compare them looking for any differences that would indicate wear or damage. They might be different too, I can't recall for your specific engine. However, bad wear or bending should be fairly visible. The fork fingers should be a loose fit in the gear slot they engage, there should be a spec somewhere in the shop manual for the finger tip thickness. The gear slot shouldn't wear (hardened steel) but the fingers will wear thinner.
The big ball bearings on the transmission shafts are a special type (because of the ring grooves) and are only available from Honda as far as I know. Since they are well lubricated by engine oil they should be OK, but the sprocket shaft one gets heavily loaded and should be checked carefully and replaced if you have any doubt about it.
Splitting the cases isn't a huge deal, and if you're doing a bunch of work anyway it's worth it to install a heavy duty endless cam chain and new primary chains.
