What I usually check:
First, under the truck. Lay down under the truck and take note of everything. See if there are skid marks on the bottom of the frame rails, look for dents in the exhaust, or bent crossmembers. Check for oil leaks, and take note of where they are coming from.
And of course, rust. You should see most of what you are going to be dealing with with the under the car search, however, I have heard a good way to check the condition of the body is to pull the tail lights out (two screws-easy) and look inside.
Open the hood, look at the engine. Take note of the timing belt cover, see if there are any oil leaks, battery corrosion (usually can tell what kind of person takes care of the truck by how much corrosion there is on the battery lol) funky wiring splices or grafts, etc.
Now, check the mileage and compare it to what you have seen. Is this normal wear and tear for 50, 80, 100, 150k miles?
Paint and interior are of course, important, however, not as important as the functionality of the vehicle.
Rock the truck body to check the shocks and struts. Drive it, see if the truck blows any steam or blue smoke. Go to a parking lot and turn the wheels hard and spin a circle in both directions in forward and reverse to check CVs. (listen for clicking). Put it in 4WD and lock in the hubs and check how it operates. Climb over a curb or something. Make sure both shifters feel tight, with no play. Take it for a freeway drive, check the alignment. Stop, put it into first and try to spin the tires.
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Check out functionality of A/C, heat, etc. Check all the lights.
When you get back, check for new oil leaks that might have appeared while you are driving.
Hope this helps.
Heather