Q: How can I tell if a software package is installed?

A: Using the vim package as an example:

dpkg -l | grep vim


Q: If I have a path to a file, how can I tell if it belongs to a
package?

A: dpkg -S <path to file>


Q: I think the system is missing an important file(for example,
stylesheets for Latex), how do I find out which package needs to be
installed? 

A: apt-cache search <filename>  or visit the Debian www site and use
their Package search page. 


Q: How do I list all of the files included with package "foo"?

A: dpkg -L foo | less


Q: Is there a GUI front-end to the package management system?

A: Yes, the program is capt. You may also use dselect although I do
not recommend it.


Q: How do I get the src to package "foo"?

A: apt-get source foo


Important man pages for Debian package management:

man apt
man dpkg 
man capt
man apt-cache


Q. I'm currently running potato 2.2.19, and looking to upgrade to
   Woody.  Is there a simplistic way to do this with apt / dpkg?

A. Change all references to 'stable' in /etc/apt/sources.list to
   'testing' and run apt-get update && apt-get -u dist-upgrade.  Hmmm.
   Isn't it better to first change the sources list, then upgrade
   Perl, then upgrade apt, dpkg, etc, and then do a dist-upgrade?
   Seems I have read that a blanket dist-upgrade from stable to
   testing is not as seamless as you suggest, but it has been awhile
   since I looked at this issue.