I always use something like
\overfullrule=1mm when I am writing Latex documents. I do read all warnings regarding overfull \hboxes, but I still prefer the visual output that very clearly shows exactly which boxes are overfull.
Can I somehow have something similar for underfull \hboxes? Some kind of simple visual highlighting that shows exactly where in the final document I have those problematic lines of text?
I am using pdflatex in case it matters.