The Overall Appearance
Does the bike look like it has been abused? Cracks in the main chassis and scratches all over the plastic parts on the bike? Appearance can be deceiving, but it should give you some indication of the general condition beyond what you can see. Look for deep parallel scratches on engine cases and on plastic (particularly above foot-peg-level); a different/non-standard paint job (the owner might have repainted it to hide accident damage); paint or metal ground off the ends of the handlebars, or the ends of the clutch/brake levers; dents in the petrol tank where the handlebars may have smashed into it during a crash ; dents and deep/parallel scratches in exhaust pipes; turn-signal stalks bent or ripped off; cracks in plastic bodywork covered by stickers. (Aftermarket stickers are sometimes used to cover defects, very common)
Is everything on the bike loose and shakey when you test ride it? You don’t need to be a mechanic to tell when the person has crashed/dropped a bike a few times. The bike should also be cosmetically symmetrical. Step back and look straight down the center-line of the bike. If something looks obviously wrong the bike has probably been crashed or fell over hard.
After the 2 points above you will be able to decide if you should walk away from this particular bike or continue negotiations with the owner/dealer.