This topic has become a serious issue nowadays. Really, you won't like all your hard work to be plagiarized, copied mercilessly just like that. But the thing that matters is that: How do we know that our blog has been plagiarized? I have been spending days looking for tools which we could use, but alas, I've found only a little. One tool that I always use to check for plagiarism is a search engine called Copyscape. This search engine allows you to type in your blog URL in the search box, and it will return a list of other sites who have the somewhat same content as yours.
Copyscape is a pretty well-known international site and if you mind the technical details, it has a PageRank of 7, and an Alexa Rank of 55,799. Don't worry if you can't quite comprehend what I said just now, just read on :).
Here is a screenshot of it:
For us normal users (that is, not premium), Copyscape only offer us a limited set of features. For example, if the specified web site has indeed been plagiarized, Copyscape will show us only a few of those site, usually about 10 sites, in the result page. To unlock its full potential, well, as you might have thought, we must PAY around $0.05 per search. This isn't much, but for casual bloggers like you and me, the free feature will suffice.