This is what I do and it works very well for me, and it's free and easy.
I make a post with the article on my blog.
I take the link from the post and pop it into the free copyscape.
Then it will tell me if there is other sites that have similar words, etc.
I have used this method successfully to assure I would not get rejected for plagiarism. And since copyscape is the most widely used checker, if you pass copyscape, you are good to go.
Then I remove the post from my blog and publish. It doesn't stay long enough to count.
Oh, and just at tip. If you are using Wordpress hosted, make sure you empty the trash so it isn't still available to the spiders.