Add the following to your .vimrc to highlight any text that extends past the 80 column mark with a subtle red background.
" Highlight past 80 columns if has('gui_running') highlight OverLength guibg=#592929 match OverLength /\%>80v.\+/ endif
It's configured to only run in GUI versions of vim since it's pretty annoying in a term. It's very useful to bind the setting to a toggle key for quick checks.
Original Source: http://stackoverflow.com/a/235970/1041926