One way is to copy a piece of the Hub and paste it into Google search and search it, and if it appears on another site then you'll see it's copied. A Hubber named Don Fairchild covers this subject in very good detail in his Hub "Monitoring Your Hub's for Plagiarism". I highly recommend the article, I have used his suggestions and so have many other Hubbers.
Edit: Of course, your question is more about finding out if content is a duplicate by chance; my answer applies to Hubs which have been stolen. I think there's still a chance of finding duplicates with this method though. I consider it a pretty good method overall to know about. There's been a strange surge, I've noticed, of Hubbers discovering that their Hubs have been stolen, and many have discovered it using this method.