The best way to find if a comment has posted somewhere else online is to search Google. If you take the comment and place it in quotes into Google search, you'll get results for where that exact paragraph or sentence is found online. Plagium will give you a message that entering just one sentence isn't enough to get a perfect result. Therefore, Google search is the way to go.

I found a way recently to discover if a comment is unique or not automatically. Install a plugin called 'Unique Comment' into your Wordpress blog. Then adjust the settings whether you want to moderate or make as spam comments that are not unique. The plugin searches Google for you and modifies the status of the comment before you ever see it. Plus when a comment is delivered to your inbox for moderation, there's a new link in the body of the message presenting the Google search for uniqueness. If Google cannot find the exact phrase, than you are good to approve based upon your other criteria.