monline

The basic method (I mean with default tools on a Debian system) is to use apt-cache:
However I've found that this does not always give correct results.

Eg. in Debian Etch for the flex package it lists the following packages: openafs-modules-source, wyg, pretzel, gcc, flexml, flex-old. But if you check the dependencies of these packages (eg. with apt-cache show package), then you'll see that only the first three are indeed depending on the flex package.
You get the correct result with another tool (that you've to install explicitly since it's not part of the default APT suite) called apt-rdepends. This one has even the ability to walk through the whole dependency tree and not just one level up or down. So use the following command:

PS: I've found the above "bug" (?) of apt-cache in apt v0.6.46.4-0.1.