We thoroughly check any submission that come through moderation for plagiarism. It is extremely rare for any page that comes through moderation to have plagiarized content. The issues arise when users gain over 1000 wiki points and are granted live edit privileges. These submissions are not checked by moderators and thus it is possible for plagiarism to appear on the site.
If you come across plagiarized content on the wiki, please report it to us via PM and we will investigate it. Please use the "PM Mods" link you'll find in the Giant Bomb FAQ sticky, since that will send the PM to every moderator and ensure that the issue is dealt with as quickly and efficiently as possible.
What about a "recent changes" section, like in most wikis? When I used to edit on the official wikis, I would always spare a couple of hours checking the recent changes for plagiarisms, vanadlism or any other edit that went against the rules. Though I don't know if can be easily coded or not.