Além dessas respostas que executam apt-cache madison <package-name>
de um terminal, podemos lançar mais luz
hob@hobgoblin:~$ apt-cache madison clementine
clementine | 1.0.1-554-geded1da~precise | http://ppa.launchpad.net/me-davidsansome/clementine-dev/ubuntu/ precise/main amd64 Packages
clementine | 1.0.1+dfsg-2ubuntu1 | http://gb.archive.ubuntu.com/ubuntu/ quantal/universe amd64 Packages
clementine | 1.0.1+dfsg-2ubuntu1 | http://gb.archive.ubuntu.com/ubuntu/ quantal/universe Sources
clementine | 1.0.1-554-geded1da~precise | http://ppa.launchpad.net/me-davidsansome/clementine-dev/ubuntu/ precise/main Sources