Many free plagiarism checkers for students are available on the Internet, including the Google search tool. It's free and does a quick and thorough job of prioritizing matches for the searcher. Just copy and paste the sentence in question and click "search."
Plagiarisma.net is operated by Google and is a another option to check for plagiarism in larger blocks of text. It supports over 190 languages. Paperrater is another free plagiarism checker that does not only check for matches within the article, but also proofreads the article for grammar and spelling errors. It also rates and gives a corresponding grade to the article. Duplichecker, while not as powerful as paid plagiarism checkers, suggests citation information of similar sites to the ones being checked.