The package showframe makes the page margins visible and hence is a handy tool if you have problems with the document margins. It will display a box showing the margins as well as lines for the header and footer.
\usepackage{showframe}
If you only need to show margins for a particular page, use:
\usepackage[noframe]{showframe}
and at the page:
\AddToShipoutPicture*{\ShowFramePicture}
The geometry package offers similar functionality:
\usepackage[showframe]{geometry}
Simple and useful.
Also consider my post on margin notes.