34,000 miles on my K1200RS. No major work or repair needed. Runs like a new bike. I do all the maintenance myself (except for the brake fluid flush every 12 months, which I will be learning about in a few weeks).
92,000 miles on my R1100RT. Replaced the clutch at 55,000 due to leaking rear main seal which contaminated the clutch plates. No other major work or repair needed. Runs like it did new.
No rear end issues at all. I run Amsoil synthetic 75-140 gear oil in the rear end and tranny, nothing thinner. I run 4 stroke motorcycle oil in the engine, not synthetic, but change it every 3000 miles without fail. BMW or Bosch oil filters. I check the engine's compression every 6 months or so, and both bikes are above "good", meaning rings etc... should be in good shape.
I believe the outward appearance will be an indicator as to how well the bike was taken care of, provided a dealer has not gone and detailed it for sale. A guy who drops and does not repair his bike, and doesn't touch up rock chips, probably doesn't do the normal maintenance either.