As we know, a post (question or answer) can be made community-wiki automatically when some conditions occur, like adding too many answers or having too much edits.
In these cases, the revision history shows an entry like this one:
Post Made Community Wiki by errordeveloper
This conversion of Better term for "user" to Community Wiki was triggered by an addition of the 31th answer by errordeveloper.
The revision history could show here:
Post Made Community Wiki by the 31th answer being posted.
For too many edits (like my answer here), it could show:
Post Made Community Wiki by the 11th edit of Paŭlo Ebermann
Post Made Community Wiki by being edited by 5 different editors.
If a moderator (or the answer's owner) actively made a post community wiki, it should show this too, of course.
(Of course, the actual phrases used here can be adapted, these are just examples.)