one or more of the following, misfires, overheating, leaking coolant, coolant running down the side of the block, could be down the back behind the valve cover gaskets, out the sides or out of the front. Get up under the truck with a flashlight and maybe even a small mechanic's mirror, that may reveal some potential issues. Also look where the guy parks the truck and see if there are stains in the driveway or road. Leaks are not uncommon on these beasts but you would rather know about them before you make the purchase so you don't have any surprises once you get her home. Good luck. Again, a pre purchase inspection is money well spent. Phil

 

I should have added to the list, strong coolant smell, steam out the back of the exhaust sweet coolant smell out of the exhaust, water rushing sound coming from behind the passenger side dash, no heat or any of the above. Phil

 

check the oil for contamination.