I have always considered this an issue for my websites and have only just found a solution to the answer. It is extremely easy and FREE!
It is so simple that I can tell you how to do it in a single sentence (infact I will try to be as brief as possible).
Take 6-8 words from a piece of content and put it into google alerts.
You will then receive an email every time someone publishes content with these 6-8 words (most likely it will be a copy of yours).
TIP: Try to avoid using hyphens, comas or other crap in the 6-8 words.
After you have found the duplicate content it is simple from here. Just hit them with all you've got. First email them and politely ask them to remove your content. If you hear nothing in 48 hours send then a dmca infringement notice, look up whois and use those contacts too, notify the registrar, their web hosting and last but not least: google.
Why pay $100's of dollars per month on some tool that detects duplicate content when you can use this method for free?