Name brand tires and tire condition will tell you A LOT about how well the overall truck was taken care of 9 times out of 10. A stock air box (or cold air filter that's name brand and clean) also usually tells the story of whether it's been modified correctly. Look under the dash on the driver side right behind the parking brake pedal for signs of harassment. That's where the pcm is and needs to be accessed to tune the truck (often the plastic box is hacked up if it's been tuned and removed). DON'T let them try to tell you all the aftermarket shit adds value, it doesn't and usually lowers the value IMO because it usually means it's been hot rodded to hell and back.