Plagiarisma is the world famous plagiarism checker for scholars, students, teachers, writers.
How to use it:
Log in https://plagiarisma.net/login.php or register https://plagiarisma.net/register.php a new account.
Grab an API key from members area and authorize your extension.
Select any text on any website.
Push the extension icon and click the "Check Plagiarism" button.
Wait for results.