Finding installed dependencies?

[cothrige <cothrige@bellsouth.net>]

I have a feeling this is a dumb question.  It seems like something that
should be relatively easy, but searching through man pages and google
has not helped me so far.  Say I have a package, 'pkgx-1.0,' installed.
Is there a way that I can list other installed packages which have that
first one, i.e. 'pkgx-1.0', listed as a dependency?  I was trying to
clean up some junk on my computer, and just would like a nice and easy
way to find out what I have which is using one or another package.

Thanks in advance,

Patrick