Посмотреть статус покета. Заодно можно узнать - установлен ли он

$ dpkg -s apache2
Package `apache2' is not installed and no info is available.
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
$ dpkg -s nginx
Package: nginx
Status: install ok installed
Priority: optional
Section: httpd
Installed-Size: 856
Maintainer: Jose Parrella <bureado@debian.org>
Architecture: amd64
Version: 0.7.67-3+squeeze1
Provides: httpd