Do you think he'll let you check the compression, I usually don't ask when buying a bike.
As long as it kicks over on the first or second kick, thats usally a good indication that the bike is jetted properly, and has good compression.
I also try to look at and inspect most of the visible gaskets make sure it doesnt look like thyre leaking, same thing on the crank case, make sure there arent any oil leaks. And no leaks again on the coolant system. Also, if you can, wobble the tires, make sure the hub bearings and axle bearings are tight.
If you can take it for a short ride, or ask the owner to, and make sure everything sounds and shifts good.