I usually just Google it:
Same thing here. I tried several different tools, but in the end I just stick to the simple Google search that proved to be the most effective way and more than in enough in 99.99% of the cases.
Google gives you raw result and you can only estimate the percent of plagiarism between original and plagiarized post.
I might be wrong here, but I don't think that percentage of the plagiarized material makes any difference when it comes to deciding whether someone will get banned or not.