Is it possible to make LaTeX mark overfull boxes in the output, for example by drawing red rectangles in the PDF where the boxes overflow? The purpose -- to more easily "see" where the problems are in the output.
As David Carlisle and egreg said, you can use the
draft option in most classes.
If you wish to do it by hand (for example, you need a
final option to check for another package's rendering) or tune it, you can use
in the document preamble.