(the package provides a demo file which contains most of the sorts of errors you might make — the example is one of them). The lacheck program analyses your source and comments on it; its view of what is “bad” is very subjective (the documentation says), but it can be useful. There’s also a web site TeXidate which will do a static analysis of your document (unfortunately, you have to paste your document source into a text window). The site doesn’t seem as comprehensive as nag, but it allows you to download its script, which you can then juggle with to make it more draconian.Package nag Warning: Command \bf is an old LaTeX 2.09 command. (nag) Use \bfseries or \textbf instead on input line 30.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=latexqual