Querying what packages are installed in postinst

[Alan Woodland <ajw2@aber.ac.uk>]

For the next version of a package which I currently have sponsored in 
Debian (mozilla-mozgest) I want to add support for mozilla-firebird 
(preliminary packages are avalible from 
http://people.debian.org/~eric/debian/i386) and mozilla-snapshot. The 
problem is that in order to do this I need to do different things for 
each of the browsers in postinst and postrm. Basicaly what I'd like to 
know is if theres a nicer way to test if a package is installed than 
just testing the existance of files that the package would create if it 
was installed?

Thanks,
Alan