1. Imaculate plastics: Could be re-sprayed or an ex-racer with the original fairings put back on. Check around for crash damage. Frame, forks or ground out pegs or bar weights. If the plastics are really clean but the rear shock and header pipes are caked in crap, be wary.
Cracked paint on subframes and fairing supports can indicate a crash as well, steel frames bend and can get re-straightened easily enough, but the paint can crack around the mounting points.

Yes, age related wear and tear can also cause paint damage, but if you see this along with the other signs of accident damage listed above it's all more evidence that suggests you should walk away.