On import. "An invalid poly-surface was read"

I just imported a .dwg model and got the message, “An invalid polysurface was read.” How can I find, isolate, and fix that poly-surface?

Maybe try SelBadObjects, and then ExtractBadSrf from it.

There’s an old but good FAQ article for finding and fixing bad objects:
https://wiki.mcneel.com/rhino/badobjects

selbad
extractbadsrf
isolate

are the commands you are looking for