what kind of mileage is on the bike? dealer or private?
what year?
Look for.
Pulsing when you apply the front brakes
Rear rocker oil leak (common problem)
Ask and see what ecm is in the bike and if its been tuned or programmed to accomadte for any modifications to the air-box, exhaust etc.
See if the guy or whoever has records of maintenance, this was a big one for me. The guy had all records from the day he bought the bike, receipts and such.
make sure you take it for a good long test ride.
Im forgetting lots but thats a start.