also...look at the seams on the engine where the cases meet the pan, block or head. If it looks like those gaskets were "visited" by a mechanic...ask if the motor has had any breakdowns or major rebuilds. Mine had orange silicon gasket sealant everywhere so it was obvious that it had been "visited" for some reason previously...so I was able to talk the price down.
Check the bearings on the wheels for side-to-side play and the swingarm too.
Pull the seat off and look to see if the wiring has been fiddled with. Do all the bulbs light up?
If you ask for a test ride and he says "NO" that could be a good sign...or he's hiding something.
Check the steering stops.
Has the engine case been welded?
Any dents in the headers?
Does it stay in gears?
Brake pads, discs and fork seals OK?
Cam chain rattle?
Front and rear wheel in line?
Chain & sprockets? Tires OK?
Shimmy at 50 mph?
Must wheelie at 7000 rpm in first on throttle alone. If it doesn't the motor is weak.
ride it, ride it, ride it! check and see if the bike shifts smoothly for sure! shifting from first to second and also check fifth gear(most people cruise around in fifth). if the transmission is sloppy, you are talking about big money! those cases will have to be split!