How can I use Drush to determine if a module is being used or not in a multisite architecture?
This works in Drush 6.x and later:

$ cd /path/to/drupalroot
$ drush @sites pm-info MODULENAME --fields=status
You are about to execute 'pm-info devel' non-interactively (--yes forced) on all of the following targets:
  /path/to/drupalroot#default
  /path/to/drupalroot#other
Continue?  (y/n): y
.path.to.drupalroot#other   &gt;&gt;  Status   :  enabled
.path.to.drupalroot#default &gt;&gt;  Status   :  enabled