Copyscape is the best site for this purpose, you can manually check the duplicity by putting two or three link content in Google in double codes.
I like copyscape. But since lots of people use it and people know about it, they discover things that people bypass. If I remember correctly copyscape only checks every 4 words/letters? Idk.
What you can do is highlight at least one paragraph of the original article and place it on G00g'3's search engine. Any website that has the same exact article will show up on top of the result and the paragraphs that are duplicated will be in bold. Try it.
Quite logical statement. People are just increasing their post counts with reply over reply. Duplicate! Now who is duplicate here?