Well for 03 and newer models some areas I would specifically check. (putting aside the obvious stuff to check, lights, radio etc) This is assuming you cant have a mechanic check it out for you.
-While driving
Take the truck up to as fast as you think you will ever drive it safely, note any shimmy, wobble, wandering etc. and have it tended to if possible.
Take lots of turns both ways and listen for any steering bind, noise etc.
Check the brakes, ABS work? Any wobble, grinding or other noise?
Engine noise, transmission slip?
Basically just be the biggest critic of the truck that you can, there's so much to look at I probably couldn't do it justice in a single post, and although not always feasable try and get a mechanic you know and trust to check the vehicle out with you.