I am writing an ebook on how to check content uniqueness on Copyscap.com I will let you know when it is out. I will give you a free copy.
DupeFreePro has a random 5 sentence check feature built in to their software that might work for a quick and dirty, free way to check for plagiarism. CopyScape is the best way to go though. If you don't want to purchase credits you could always load your article on a Blogger blog and submit your blog post URL to Copyscape, then pull your blog post down. But really the CopyScape costs are so cheap it's practically free anyway.