I’ve found that the best way to spot plagiarism is to have your students write often and know their writing styles and abilities. I can easily spot plagiarism because I know my students well. I then check the passage or passages with any number of free plagiarism checkers online and also Google search sections that seem particularly suspicious!
The best deterrent for all of this is to teach students about plagiarism at the beginning of the year and show them how you are going to check over their work and how they can check their own work before submitting!
Oh…and here is an article with some more plagiarism checker options and thoughts!