Service records. At the very least, I'd pull an oil sample and send it off to Blackstone in Fort Wayne and get an oil analysis done before buying. That will tell you conclusively if there are internal engine issues. If it's an automatic, I'd do the same with the fluid, even if it don't appear burnt (brown and stinks if burnt).
If the owner balks about an oil analysis, I'd walk away because something is fishy.
Oil analysis keeps everyone honest.