With layout package
As @Speravir said, the layout package does exactly this: it prints the page layout, with all lengths. This is the best answer, since this package is part of LaTeX tools bundle, therefore "a part of all great TeX distributions".
Here's a MWE:
\documentclass{scrreprt}
\usepackage{layout}
\begin{document}
\layout
\end{document}
And here's the output:

With layouts package
The layouts (plural) package extends the funtionality of layout, allowing you to do much, much more. To begin with, you can print the page layout alone (with \pagelayout) or just a table with the values of the page parameters (with \pagevalues) — but it does much more than just that. \pagedesign gives a simplified version of the layout package's \layout command.
\documentclass{scrreprt}
\usepackage{layouts}
\begin{document}
\currentpage
\pagedesign
\end{document}

And a combination of the commands mentioned before gives an output similar to the one given by the layout package:
\documentclass{scrreprt}
\usepackage{layouts}
\begin{document}
\currentpage
\pagediagram
\pagevalues
\end{document}

1in + \oddsidemargin. – Ulrike Fischer Apr 10 '12 at 14:45