From time to time I log onto Copyscape.com to see if anyone has copied text from my website ... they offer both a FREE service (as well as a premium one that's very reasonable for a more in-depth search). You simply enter the url of your websites's various pages and instantly receive a report on places where that text is shown. You'll see that it includes your text on sites where you placed it (like ActiveRain, your own website, Linked In or other profiles, etc.) as well as sites where PLAGIARIZERS have copied YOUR content and are using it as their own.
Once you click on those "unfamiliar" links, Copyscape.com gives you a full report that shows the Source Page (YOUR content) as compared to the COPYCAT'S page. Your text will be highlighted and so is YOUR text on THEIR page, so both you and the person who stole your original content can easily see what's been copied. Copyscape.com also includes a Word Count to make it very clear how much of your content was copied. For something that's this helpful, it's amazing that it's also FREE.
If you'd like their Automatic Search service, that costs a bit more but is the easiest way to make sure your original content is safe from copyright infringement.
As a professional home stager in Portland, Oregon I've been dismayed to discover that home stagers from around the country have copied my website text. When this happens, I email them a polite but firm letter giving them the link to Copyscape.com's report as well as a demand that they remove my text from their website within 48 hours. If they ignore my request, they receive a more firmly-written letter that references the Digital Millenium Copyright Act, Title 17, section 512, US Code (commonly referred to as "DMCA").
A final resort is a letter from an attorney detailing the financial penalties they'll be liable for if the plagiarized text isn't removed immediately. You can also contact the host of that person's website and request that their site be taken down -- I did that last week and the offender's entire website was shut down in less than 24 hours -- because she'd refused to remove my text from her site. Fines and penalties can be surprisingly stiff since federal law protects original content.
*Oh -- and you COPYCATS? Don't offer that lame excuse that your "webmaster must have done it" -- we've heard that before and as the business owner, you're the responsible party for making sure that any content on your site is YOUR OWN.