Pop the seat and look at the fan. If its oily, you'll need rocker gaskets. While its off, look at where the passenger foot pegs bolt on, if they're cracked it's been down…
Look underneath where the kickstand bracket bolts on. They like to break the case if wrecked.
Check the brake pads with the flashlight you brought
Can you see under the frame pucks? They might be hiding a dent.
Take a voltmeter to check the battery voltage when cranking and after running. Check the date code on the battery.
Check for codes.
Have someone roll it forward while you look at the tires for plugs, or patched holes. Listen to see if the wheel bearings or idler are making noise. Then use the front brake to stop it quickly. A "clunk" means steering head bearings.
Don't assume any lights out are just burned out.
Do all of the warning lights come on while the gauges sweep?
Push every button, switch every switch.
Check all the fluids, and smell the oil.
Make sure it's cold and not previously warmed up. It's ok to warn him you'll be expecting it cold.
Match the owners ID to the pink slip, hand him all your cash, ATGATT, and go for a 15 min test drive. Don't be stupid, warm it up nicely. But hard stops and high load/RPM shifting to check the clutch and suspension.
The whole process should take 30mins, all your doing is popping the seat (not disassembling the bike!). If they aren't willing to let you see anything or take a test drive, then you should take a walk.
Can you tell I've bought a buch of crap bikes over the years??