Personally, being a command-line person, I would do something like:
LCMD SHOW X-LISTKWD OWNER ALL | grep OWNER
Then split and sort, or something like that. You would have to use lcmdx instead of LCMD on unix.
Like all X- commands, this one is not meant for direct human use and is instead formatted so it can be parsed easily by scripts. It is near-instantaneous as Francoise said, but in earlier versions of LISTSERV without this command, the report Francoise mentioned did take a while if you had many lists.
Eric