chroot into it, and run dpkg would be the easiest thing. See http://superuser.com/a/417004/20798 for how to get a working `/proc`, `/sys`, and `/dev` inside the chroot.


Since you have a working debian system outside the backup, you could probably just use

 dpkg --admindir=dir --get-selections

The `dir` defaults to `/var/lib/dpkg`, so put the path to your backup's `/var/lib/dpkg`.