^^^those holes in the frame in the picture above are for if the vehicle was in an accident and the frame was bent, it will go to the frame guy to use hooks in those holes to restraighten the frame, then bolt everything back together, so if those holes look torqued in any way you can probably bet it was in a bad accident.
Also, check out the engine, if it looks oddly "shiney" from washing and pouring tons of cleaner on it...it might have some sort of leak or was off roaded really hard and theyre trying to cover up something.
Also, check bolts on the fenders/hood and anywhere that the body bolts would be painted. if they have chipped paint/paint around it is wrinkled..etc it may have had some damage and needed to be replaced/fixed, if not, then most likely it hasnt been wrecked.
Look at all the panels on the truck, all body lines should line up with each other, doors/tailgate/hood should all open without trouble..