From http://bugs.musicbrainz.org/ticket/5619

To help people on #musicbrainz with debugging issues like bugs.musicbrainz.org/ticket/5609 it would be very useful to include the port on which picard is listening somewhere in the interface.

(though I personally think it is an interesting thing to display regardless

A good place could be the status box on the bottom right, something like:

"Files: 22, Pending Files: 12, Listening on 8000".

Though anywhere in the interface will do, you could for example hide it on the help->about screen or something.