Displaying page construction guides

Just to add another useful package that does it:

\usepackage[showframe]{geometry}

(as many people load geometry anyways, it is a nice gadget)

Use \layout form the layout package. This produces:

and prints the layout details on the bottom:

\documentclass{article}
\usepackage{layout}
\begin{document}\layout
\end{document}

If you just want the boundaries overlaid on the actual pages, independently of other packages,

\usepackage{showframe}