rear quarters, truck floor, floor pans, torsion bar frame. Those are what comes to mind right off the bat.

 

look at the engine: leekes, rust, etc

 

if its got a vinyl top check under that if possible for anything major especially along the front of the roof and pillars

 

Top of the front fenders over the wheel wells and just behind the headlights. There are places there that trap the dirt and moisture and cause rust too. On the firewall, check the drain hole from the wiper tranny area, it's another area that gets clogged. Under the battery box, and around the hood hinges.