Welcome aboard, Adamastor, glad you found this site, pretty even tempered bunch here !!
A good general look at the bike, fluids leakage, puddles or oil stains on the pavement under the bike, is the engine, transmission, swing arm, final drive covered in grime, a good indication of some serious leakage .
Oil leakage on the fork seal area .
How much tread groove left before the wear bars show on the tires .
Check the steering head bearings for notchiness, standing in front of the bike, with the bike on the center stand, grab the front wheel, and see if there is any play, when you pull and push on the front wheel .
Smoothness of the throttle, 'choke', and clutch cables .
If you can have the seller remove the fuel tank, and have a look under there, is the ignition coil a gray and black model, if it is, it's in your best interest to replace it, as soon as you can, it's one item, that will leave you on the side of the road, especially in wet or damp weather .
Gear oil leaking from the final drive output seal, will leave a splattering of oil on the rear wheel area, and compromise the effectiveness of the the rear braking, and probably have you replacing the rear brake shoes as well .