While there are several ways to use the Internet to check for plagiarism, a common and simple way is to enter sections of the document into a online search engine such as Google, Bing or Yahoo and see if any matches are returned. Putting quotation marks around sentences or phrases narrows the search, potentially revealing any segments of the document that have been copied verbatim. This approach is most useful when one or more sections of the document being reviewed seem out of place.
Electronic paper review services such as Turnitin and iThenticate are other ways to check for plagiarism, suggest Plagiarism.org. Turnitin is used in academic environments, primarily for reviewing undergraduate papers upon their submission. These programs require students to submit their papers online. The program then scans the document and returns a report detailing what percentage of the document is found to match with others. Usually a "plagiarism threshold" is established--using a percentage of matched phrasing--for determining if a paper is considered to be plagiarized or not. There are also some free online programs, such as Plagiarism Checker.
Internet services and searches are limited to what is found on the Internet or in a program database, so not all plagiarized material may be detected by a digital search. Another way to check for plagiarism is to search sources--especially those assigned for a class--to see if any passages in the paper closely resemble published passages. This process can be time-consuming, so it's best reserved for rare or exceptional sources that are unlikely to be found online.
In the digital age, face-to-face interviews are often neglected as a way to detect plagiarism. But talking to a student or author is one direct way to ascertain if any plagiarism has taken place. Not only do mannerisms or straight-out admissions of guilt often give away plagiarists, interviews allow the reviewer a chance to ask the author some basic questions about their work. If the author is unable to answer, or if the answer fails to match their written submission, then plagiarism may have been committed. This is a good tactic to use in tandem with others, such as an Internet search.