chroot into it, and run dpkg would be the easiest thing. See https://superuser.com/a/417004/20798 for how to get a working
/dev inside the chroot.
Since you have a working debian system outside the backup, you could probably just use
dpkg --admindir=dir --get-selections
dir defaults to
/var/lib/dpkg, so put the path to your backup's
Don't forget that
dpkg --get-selections doesn't show which packages were manually installed, and which were only installed to satisfy dependencies (and thus should be auto-removed when no longer needed because newer versions of the packages you actually want have different deps, or because you purge a manually installed package.)
I use aptitude, which makes it easy to mark everything as auto-installed, then go through and mark some packages as manually installed until nothing you want to keep is getting auto-removed. Start with big meta-packages, like
build-essential, the Debian equivalents of
ubuntu-desktop, and stuff like that. In aptitude, hit
r to see the reverse-depends of a package (pkgs that depend on it).