The most common areas for frame rust on a 40 series are the gusset plates above the rear-most spring hangers, the rear crossmember, and the diagonals that run from the frame rails to the crossmember. As long as the frame rails themselves are okay, most of the gussets/brackets can be replaced. If the steel looks like its "delaminating," it is shot.
On pre-1981 trucks, the frames also tend to crack where the boxing ends above the front-most rear spring perches. The crack tends to run longitudinally from the rear-most rivet on the perch towards the back of the truck. The spring perches can also be tweaked, especially if the truck has been running extended shackles. As you sight along the framerails the shackles should be perpendicular to the bottom flange. Tweaked fixed-pin spring perches are rarer, but if a rear one is out of alignment, chances are the frame has cracked. Because of the stronger design, perch/hanger problems aren't as common on post- '81 40s and 42s. Feel through the holes on the inboard sides of the frame rails. Large flakes of rust are a sign of problems to come.
60s and 70s only real problems stem from rust. Their frame designs don't have the same large holes as on 40s so they're harder to clean and more prone to trapping crud and rusting. Feel through the small access holes for signs of rust flakes. The worst spots for frame rust on 60s and 70s are where the muffler is next to the frame. It is possible for the steel to be weakened to the point that a screwdriver can easily be poked through it with no outwardly visible warning signs. The other big problem area is the rearmost section of the frame where it is just a channel (as opposed to fully boxed)