Waterlogged bottom bracket areas are the main concern.
I drill holes in the bottom of the bottom bracket shell for this reason. The tapped M5 hole for the bolt to fix the derailleur cable guides is insufficient for drainage, they really ought to all have a second hole, even if it is obscured by the plastic cable guide.
The chainstays near the bottom bracket will rust, as will the lower portion of the down tube. Rusty BB threads and smattering of rust dust in that region will tell you whether to investigate further.
First step is capping the BB openings and pouring liquid down the seat tube, then swooshing it around. If what drains out after removing the BB plugs is more the consistency of mud than of tainted water, or contains flakes, I’d ditch the frame.
The next step is flushing the frame of any loose rust, and thoroughly drying it out.
After that, tap the region of the down tube around the BB junction, repeatedly, at different spots, with a heavy solid object, firmly enough to risk denting the tube. Don’t hit the chainstays … the rust there will be equivalent to the bottom of the down tube. Shake the frame after. If rust flakes dislodge, ditch the frame. You’ll get a feel for what’s going on by the pitch. Hollow thuds, rather than pings, tend to precede flakes. Really bad frames, you’ll tap a hole right through the frame on the first hit.
Flakey paint often indicates water damage, but pristine paint doesn’t guarantee absence of internal rust.
In short. Dusty rust, not much concern. Chunky stuff, ditch the frame.