Is there a good tool to check plagiarism?

Can we detect plagiarism 100%?

The internet is one of the best tools ever developed for tracking obscure references and quotations. Copying any passage that doesn't "sound" like a particular student and pasting it into a search engine topic bar can find even out-of-print quotes from the student's grandfather's bookshelf.

Your second question about 100% can only be answered negatively. It's not 100% certain that I'm answering this question; I sometimes have very vivid dreams.