The first thing to do is a very, very close inspection. Under the truck you should be looking for leaks, loose broken wires, tubbing. Look for signs of abuse, like dented exhaust,cross members, drive shafts. Check out the exhaust, it probably needs replacement, and this is a leverage point for a price reduction. Was it used for heavy towing, look for signs of an old gooseneck/fiver hitch. Check the shocks, are they leaking oil? are they the originals? Tires, are they cheap -o replacements? are they the right size?
Under the hood, look for general appearance. Does it look right. Many times when a dealer does a quick power wash, and coats down everything with the tire dressing, they are trying to cover things up. Like old hoses, belts, ect. What type filter does it have? Oil has probably been changed, but look anyway. The one ITEM YOU DON'T WANT TO FORGET is the auto trany. After it is warmed up and run thru the gears, check the level. Look at the color, is it bright red?? If it is dark in color, smell the fluid, does it smell burned?? If so plan on a trany soon, again this could be more leverage on price. But remember tranys are expensive, but in all honesty, and truck that is older then 3 years, or over 60,000 mile I would be concerned. Trany's are the week spot automatic or manual, they both have problems. On the inside of the cab, look at the pedal, are they worn badly? does it look like they match the miles indicated. Some dealer will throw on new rubber covers, watch those. Also the worn rubber pads on a newer vehicle indicate much stop and go, like maybe a delivery truck, stay away from them. Make sure all the controls are functional, try each one. Ac repairs are expensive so try it now.
On the test drive feel for any odd vibrations, does it pull bad to one side, how about when you brake? How are the brake?? does the emergency brake work?? check it. Put the transfer case into 4x4, does it engage smoothly? does the 4wheel drive indicator light come on? Can you feel the front tires when in 4 wheel drive? Transfer cases are very expensive, so make sure.
The most important thing is for you to trust your "gut" feeling. Usually our instincts are right on, so leave the emotions out of the purchase, and look with both eye wide open. Good luck