Hubpages has a feature built in that let's you know when your work is duplicated online. This will be under your account summary. You can also use free plagiarism checkers if you are suspicious of someone taking your work.
Someone on Hubpages suggested this link to me so I will share it with you. It definitely works! My favorite part about it is it will check the search engines to see if someone ripped off your meta description/summary.