vigilance is the answer. When you find that your content is being used elsewhere, an appropriate first step is to contact the site and let them know.
Ideally at this point, a known content contributor is responsible and the website’s management was wholly unaware (and will gladly take down what is not rightfully theirs).
If that option is exhausted and you still have an issue, however, the search engines need to be made aware. Use Google’s Online Copyright Infringement form to establish yourself as the rightful owner of the content in question; doing so will protect your site from SEO penalties related to that content.