Ball joints. tie rod ends. Oil leaks, smell for rad leaks,
Listen to the engine for misses and odd pings. Check the oil for colour (milky is bad) look at the dipstick for rust on it(not good rust on a dip stick) look at the rad fluid for oil floating it in.(also not good)
Look at the exhaust for white/blue/black (white=coolant, black = running rich, blue=burning oil) smoke after the engines warmed up. small amounts of blue smoke on starting and then going away is generally valve seals.
Look at the frame get right under it and tap any bad looking sections see it it still has a nice steel sound or is going crunch, crunch is bad btw. knocking a outer layer of rust off and then hearing a good steel sound is ok.
Check the springs for cracks, check the leaf hangers for bad rust the same as the frame. its a common rust through point.
Check the cab corners for rust. don't beat on them like the frame its much lighter duty steel and won't take the abuse. Just look at it if you see peeling rust its not a great sign. same with the floors. and the bed. rusty bed fenders looks bad but is a minor problem that can be fixed when you have nothing else to fix LOL.
Look at brake lines again looks for rust same with fuel lines. If someone has replaced sections of fuel lines with rubber high pressure line(which is good) there maybe still sections in need of replacement.
Check tire wear patterns, bald on one side or the other indicates needing a alignment. which can also lead back to ball joints and tie rod ends.
Thats all I can think of for now