It's actually quite simple, and this can be used to also track any package that was installed on your system from a different "origin". The "origin" field is defined in the mirror's Release file and represents where the packages come from, be it Debian, Ubuntu, or deb-multimedia (example: here is Debian Wheezy's official Release file).

You can check for non official packages in Aptitude using this syntax in the search field, triggered by the / ("slash") key:

!?origin(debian)

You can also list all the packages matching this pattern using the "limit" functionality, through the l ("ell" like Lima) key.

If there are matches and they are not obsolete packages, you have a tricky operation to perform. deb-multimedia was hacking at the epoch to force installation on top of official versions, a really bad practice that was one of the strongest arguments against deb-multimedia in my mind. The epoch, basically, bypasses regular version checks so, in the above linked here, if you want to upgrade from deb-multimedia's 1.0 vlc to the official 2.0 vlc, it's actually a downgrade because of the epoch. So in short, switching from deb-multimedia actually appears to be a downgrade, but it's generally not.

To wrap it up, to migrate away from deb-multimedia:

  1. remove it from your sources.list files
  2. fire up aptitude -u
  3. list the unofficial packages: l ("ell") then !?origin(debian)
  4. for each unofficial package, hit enter to see the detailed description, scroll down to the very bottom and install the other available version, with + ("plus")

To tell the two versions apart, look at the version number. If there is a 1: prefix, that is the nasty epoch. The official packages usually don't have those.

If you are still hesitant at ditching DMO, keep in mind that deb-multimedia is maintained by a single person while there is now a whole team maintaining multimedia in Debian. By using the official packages, you are helping Debian in testing those official packages. And besides, you can always go back and reinstall the deb-multimedia sources if you prefer.

The Debian Multimedia team also maintains a FAQ about DMO which explains the difference between the two repositories. See also the Debian-multimedia.org considered harmful thread.

You likely want "^debian$"
You likely want "^debian$" instead of "debian".
Comment by Julian Andres Klode
If you are still hesitant at

If you are still hesitant at ditching DMO, keep in mind that deb-multimedia is maintained by a single person while there is now a whole team maintaining multimedia in Debian. - now that's strange, you are telling me that single person is doing a better job than a whole team? Because apparently this is the reason why people use deb-multimedia at all.

Comment by Anonymous
Thanks for taking the time to
Thanks for taking the time to write this. After some reading of the mentioned debian-devel list thread, I got convinced after one of the mails.
Comment by Marco
no
er... no, that's not what I am saying at all: I am arguing that the fact that only a single person is maintaining DMO is a problem. It makes a single point of failure, and I do not believe (anymore) that DMO is better maintained than regular multimedia packages in the archive.
Comment by anarcat
Just did so.
I just did, not sure if it's the right thing to do, but it does make some sense to me right now.
Comment by anarcat
Created . Edited .