You can inspect the fifth gear with just the cilinder removed. Then, from the front of the engine, look inside towards the back and you will see the gears. Take a look at microfiches to determine which ones are fifth gear.
However, the clearest sign of worn gears is a sound like a dive bomber when you drive along a concrete wall. If you don not have that, there might be some pitting but your gears wil make the trip no problem.
To reduce load on the gears:
- Never ride below 4500 rpm
- Gear down: one teeth less on the front sprocket, or 2 - 3 more on the back
Check the nuts on the crankshaft and balancer shaft and torque them to correct setting. Replace the key on the balancer shaft (this last one: on some bikes this nut came loose, balancer hit the conrod -> total disaster).
Clutch plates ok?
Oil pump ok?
Take some spare seals for the output shafts: original Yamaha, not expensive, easy to replace when necessary.