If you live in the rust belt, obviously the undercarriage, and all suspension components, if they too rusty (as in everything is covered in rust, a little here and there is probably not a big deal), or you can find rust holes anywhere (exhaust is the only exemption), walk away, if the undercarriage is not rusted see if it runs smoothly, understand that more than likely you will have to do all the maintenance (timing belt, water pump, seals, etc along with fluid changes, etc) turn on the AC, heat, operate all the electrical things you can find, and see what works and what doesn't, look for mismatched panels, check the oil and coolant before you turn on the truck, if you can crawl underneath and see if the crank pulley has the new bolt or the old version (look it up), operate the seats to make sure they work, if it has the orange sticker for the rear locker try and operate it, operate the 4x4 system and see if there's any slop on the shifter, again turn on lights, stereo, etc, check the lights on the dash for flickering which is a alternator that has a failed voltage regulator, ask about the history of the vehicle, and calculate the price you are willing to pay.