Determining if package installed

[LaMont Jones <lamont@hp.com>]

I have a package (Postfix) that wants to behave differently if NIS is
installed (more to the point, if there is an NIS alias map).  Since I've
managed to remain almost completely clueless on NIS, I was hoping someone
could tell me the best way to see if NIS is installed, or scheduled to
be installed.

Then, of course, there is the question of the best way to tell the user
(who probably has no intention of installing NIS) that if he ever does,
he'll need to edit the Postfix config file if he wants a NIS alias map
to be used...

comments, flames, etc?
lamont