you can't really inspect anything without opening up the gear box, and that has to happen with it out of the bike.
I just had mine rebuilt and it was $550 labor, and i needed $450 worth of parts. If I had only had it "inspected" - stopping after the case was opened to see how bad it was, I still would have had to pay a couple hundred $'s. I believe all the parts are available new. or at the very least, used.
some symptoms would be lots of metal on the magnetic plug, lots of bearing noise, difficulty shifting...
Hopefully some actual knowledgeable people will chime in:)
1978 BMW R80/7
"all we need is time and courage." - Renner
"A lightened flywheel on your bike is like me getting into Spandex and putting on track shoes to walk the dog." - M