I think the sheet metal has been beaten to death so I'll add a few more:
Electrical - as with any car make sure everything works, get under the dash and check to see how hacked the wiring harness is. Look at the quality of work for things added in (stereo, aftermarket horn, etc) by the PO. Electrical demons are a royal pain unless you KNOW exactly what you're doing (like me
). Check the engine bay for loose wires, crappily done surgery, wiring routes (like the rad support channel and firewall channel)
Engine, obviously check if it leaks, knocks, smokes, backfires, etc. Be wary of the engine being warm when you get there unless its been sitting and not been driven around as a daily driver.
Transmission - make sure it shifts good at various throttle ranges (auto) if its a manual be wary of clutch slippage and meshing gears/noises (bad syncros). If its an auto don't be afraid to pull the dipstick and smell the fluid - avoid burnt smell.
Check the frame thoroughly in a northern/coastal truck, thru rust on the frame is NOT uncommon and can cause very very bad things!
Lastly, check for the small stuff. Cracks in lenses and running lights might be a small thing, but it costs money to replace and you don't want to be nickel and dimed to death.