check all the moving parts, clutch brakes, throttle, make sure it all feels good, check for moving side to side in the sway arm(that could mean new bearings, check for movement side to side in the back and fount wheels check for leaking oils/fluids on the bike and if u see any thats probably not a good thing,also make sure the bike isn't hot by feeling the engine and pipe and if it is it may have cold starting issues so while you look at other things and talk to the owner let it cool down then start it, a thing you can check at home is a clean air filter but usually don't buy the bike then take it home and check all these things, don't by shy to go through all the gears, and i usually change the oil and oil filter the day i take it home, better safe than sorry