Checking for installed package-alternative in a wrapper-skript

Hello,
Is there some canonical fast way to check in a script whether inn,
inn2 or cnews is installed on the system?

I need to know it in newsx's wrapper skript to set the correct
options. Until now I've used
if [ -e /usr/share/doc/inn/copyright ] ...
which works well, but breaks policy 13.3. I would have liked to check
for existence of /var/lib/dpkg/info/$package.list, but this'll still
be there if the package is deinstalled but not purged.

I'd rather not check for some files in the package like
/usr/lib/news/input/newsrun vs. /usr/sbin/ctlinnd because they tend
to be moved around, and calling dpkg'd be slow.
             thanks, cu andreas
PS: Please Cc me on replies.
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