Require all of your students to submit their papers to a plagiarism detection service like the one offered by Turn It In (turnitin.com). Turn it in is one of the most sophisticated plagiarism detection services and many colleges and universities have established accounts with the service in order to deal with the issue of plagiarism. The advantage of this website over other services on the Internet is that it checks papers for plagiarism against Internet sources and all other academic papers that have been submitted by other schools. This can help you detect plagiarism from Internet sources and from papers that may have been purchased and submitted at other schools.
Search for plagiarism using other Internet-based detection tools if your school does not have an account with Turn It In or another academic search service. Set up an e-mail account that you can have students send their papers to as an attached file and require them to submit both a paper copy and a electronic copy of their paper before you grade it. Papers that appear to be suspect can then be accessed quickly through your e-mail account. Copy and paste the entire paper to an Internet-based plagiarism detection tool like Article Checker. This will produce similar results to what you would obtain with Turn It in, but without the reliability of a check for previously submitted papers.
Google suspect phrases by using "quotation marks" around the entire phrase or part of a paragraph that you consider to be questionable. Use this method if you don't want to take the time to set up an e-mail account or don't have access to a site like Turn It In. When inputting your information into the search engine, use the entire sentence to obtain the most accurate result. Use two back-to-back sentences for even more accurate results. If a student has taken the time to copy one sentence, it may be likely that they have copied two or more.