(from my comment to this answer)
Another suggestion is to use a screen ruler on top of your .pdf document. For Linux, I strongly suggest ScreenRuler:
ScreenRuler is a small GNOME based utility that allows you to measure objects on your desktop. It can be used to take both horizontal and vertical measurement in 6 different metrics: pixels, centimeters, inches, picas, points, and as a percentage of the ruler's length.
In Fedora, I know it's easily available through yum install screenruler. In Ubuntu is probably the same.
Consider the following example:
\documentclass{article}
\usepackage[top=1cm, left=3cm, right=2cm]{geometry}
\usepackage{lipsum}
\begin{document}
\lipsum
\end{document}
After compiling it, I opened the resulting .pdf file and ran screenruler:

As DevSolar mentioned in the comments, if you zoom your file, you need to manually set the dpi resolution. :-)
pfdpagesthis will help you obtain the same result. – Peter Grill Sep 23 '11 at 19:21:-)– Paulo Cereda Sep 23 '11 at 19:39.pdfdocument.:-)I didn't write it as an answer because, in fact, I didn't answer the original question (a feature on a PDF viewer), but provided an alternative.:-)– Paulo Cereda Sep 24 '11 at 00:21