Plagiarisma is a great site to try and check to see if an item is being plagerized. This can be done by either pasting in a URL, uploading a document, or pasting in text. The results are then given as URL's that have similar data. Also, this can be a great teaching tool to show students why not to plaigerize.
I highly recommend educators to check out Plagiarisma to see how best to make Plagiarisma work for them.