Enter your domain name, select which version is your primary domain (with the WWW or without it) and submit. The tool will compare the number of pages indexed for a set of searches in Google to check common duplicate content problem areas. If it finds any it will tell you what they are, how bad they are, and how to fix them. So give it a whirl and tell me how you like it, just please don’t try to break it too hard.
I talk more about these duplicate content problems in my upcoming book, too.