List of all installed packages in a defunct Homebrew installation? I have an old homebrew installation that is now not in use on a backup drive.  I'd like to list all of the packages that were installed when it was in use, how can this be done without mv'ing it back over to /usr/local and making it operational again?
According to the Homebrew terminology, we have the following definitions :

Cellar :  All Kegs are installed here
Keg :      The installation prefix of a Formula
Formula:   The package definition
Homebrew installs to the Cellar it then symlinks some of the installation into /usr/local so that other programs can see what's going on.

As said in the comments, to list all packages previously installed on a backup drive you can do :
ls -1 /Volumes/*/usr/local/Cellar

If you want to have a better understanding of Homebrew paths, I recommend the reading of the following article : The Path to Homebrew.