this should get you a list of all the users which have been disabled by means of `usermod -L`:
perl -e 'open($SHADOW, "<", "/etc/shadow") or die( "$!\n" ); while ( <$SHADOW> ) { chomp; print "$1\n" if (/^([^:]*):!{1}[^!:]*:.*$/) } close( $SHADOW );'
you'll need to run it as root. no doubt that regex could be cleaned up a bit :)
thanks both - that gives me something to work with