apt-cache show on Debian/Ubuntu gives you a decent amount of information about an installed package. I have used this before, but not consistently.
e.g.
$ apt-cache show python
Package: python
Priority: important
Section: python
Installed-Size: 658
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Matthias Klose <doko@debian.org>
Architecture: amd64
Source: python-defaults
Version: 2.7.3-0ubuntu2.2
Replaces: python-dev (<< 2.6.5-2)
Provides: python-ctypes, python-email, python-importlib, python-profiler, python-wsgiref
Depends: python2.7 (>= 2.7.3), python-minimal (= 2.7.3-0ubuntu2.2)
Suggests: python-doc (= 2.7.3-0ubuntu2.2), python-tk (= 2.7.3-0ubuntu2.2)
Conflicts: python-central (<< 0.5.5)
Breaks: python-bz2 (<< 1.1-8), python-csv (<< 1.0-4), python-email (<< 2.5.5-3), update-manager-core (<< 0.200.5-2)
Filename: pool/main/p/python-defaults/python_2.7.3-0ubuntu2.2_amd64.deb
Size: 167552
MD5sum: 81b302b2f98d8125fa60a886a61644fe
SHA1: 336fcd82246e00f52455112dba780fa34cb0eaa8
SHA256: bec692c0b1629e68002b317d3bcef0e6234d723d7aafcec5e0d81eb94449b76c
Description-en: interactive high-level object-oriented language (default version)
Python, the high-level, interactive object oriented language,
includes an extensive class library with lots of goodies for
network programming, system administration, sounds and graphics.
.
This package is a dependency package, which depends on Debian's default
Python version (currently v2.7).
Multi-Arch: allowed
Homepage: http://www.python.org/
Description-md5: d1ea97f755d8153fe116080f2352859b
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
Supported: 5y
Task: minimal
Package: python
Priority: important
Section: python
Installed-Size: 658
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Matthias Klose <doko@debian.org>
Architecture: amd64
Source: python-defaults
Version: 2.7.3-0ubuntu2
Replaces: python-dev (<< 2.6.5-2)
Provides: python-ctypes, python-email, python-importlib, python-profiler, python-wsgiref
Depends: python2.7 (>= 2.7.3), python-minimal (= 2.7.3-0ubuntu2)
Suggests: python-doc (= 2.7.3-0ubuntu2), python-tk (= 2.7.3-0ubuntu2)
Conflicts: python-central (<< 0.5.5)
Breaks: python-bz2 (<< 1.1-8), python-csv (<< 1.0-4), python-email (<< 2.5.5-3), update-manager-core (<< 0.200.5-2)
Filename: pool/main/p/python-defaults/python_2.7.3-0ubuntu2_amd64.deb
Size: 166114
MD5sum: 5e0615e173a3834d0b2e97768bfe4d2c
SHA1: a748ba4f38c3dd94bc6d782a8421499b01c406f3
SHA256: ec8a24ec3cf3dfdf42581fa529e0ee69dc08bd8c29aeb601c329c337e5605e0e
Description-en: interactive high-level object-oriented language (default version)
Python, the high-level, interactive object oriented language,
includes an extensive class library with lots of goodies for
network programming, system administration, sounds and graphics.
.
This package is a dependency package, which depends on Debian's default
Python version (currently v2.7).
Homepage: http://www.python.org/
Description-md5: d1ea97f755d8153fe116080f2352859b
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
Supported: 5y
Task: minimal