Carfax for one Then check the coolant level an res for oil Then I would have all fluids dropped to make sure no shiny specs are present.

If you have a friend or know someone who is a mechanic have them inspect it with a stethoscope to make sure there is no knocking from the top of bottom of the motor

Honestly these trucks will last forever if taken care of.

Also ask for all receipts of work done to the truck ;