Under copyright law in the United States and in many other countries, copyright is automatic as soon as an author creates an original work. This includes content posted on the Web, where copyright infringement is rampant. Whether the content is stolen by a person ignorant of copyright law or one who does so deliberately, it is still illegal. Checking online for copies of your work occasionally will protect your content from potential misuse.
Familiarize yourself with copyright law if you don't already have an understanding of the subject (see links in Resources). Knowing your rights as author of a work will help you defend it if it comes to that. A grasp of the law will also help you to recognize what acts constitute copyright infringement. Be sure to review the Fair Use aspect of copyright law as well.
Run your content through a plagiarism checker. For text, you can check your work through a website such as Copyscape or simply paste quoted portions of your text into Google or another search engine. For images, use a reverse image look-up site such as TinEye or Google Chrome Image Search (links in Resources).
Analyze the results of the search. All of the search methods mentioned above will return copies of what you searched. Look through the listings and pull out any results in which a significant portion or all of your work was copied. Make screen shots of the offending Web pages.