You should look at maintenance receipts to ensure that everything was done on time and is up to date. Specifically look for any gaps in service. This may be a sign of 1 of 2 things:
1. A service was missed. Might not be the worst thing, but it makes you wonder if the bike has been properly treated or was otherwise abused.
2. Bike was seriously dropped/crashed and while the forks were being replaced, they went ahead and changed oil, set timing, etc. This service receipt might be missing for obvious reasons — not good.
Check the bike for any signs of being dropped. Newer mirrors or grips, small fairing cracks, mismatched turn signals — all are signs of the bike being on its side, possibly crashed. These are not reasons for rejecting the bike, but should prompt additional questions and inspection — maybe to the point of having the seller pull some fairings.
What you should hear is tough. The sound should be smooth, and there should be no sever rattling. Have someone take the bike for a ride and put it through ALL the gears. Make sure it holds 2nd under stiff acceleration.
Taking an experienced friend along can be really valuable. A mechanic check is also good insurance.
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