If you suspect that a student has plagiarized a sentence, the simplest way to check is to enter the sentence into a search engine such as Google, in quotation marks. Any online article containing the sentence will be brought up by the search engine. There are also free online tools, such as The Plagiarism Checker, which can search for larger chunks of text in one go.
Some plagiarists attempt to mask their deceit by making minor changes to the sentence structure or wording of the text they have appropriated. If a Google search as detailed in Section 1 doesn't reveal a direct match with another Web page, remove the quotation marks. This will bring up a list of online documents with similar wording to the student's work. You can then check the work for similarities with these sources. Where content has been plagiarized from more than one source, it might become apparent that there are differences in writing style within the paper. Check the paper for anything odd, such as page numbers out of place, references that are not cited in the bibliography or for information that seems to be out of date.
Be aware of the existence of "paper mills." In November 2006, researchers at Coastal Carolina University found more than 250 websites dedicated to selling pre-written essays to students. Compare a student's suspect paper with previous work they have submitted. If it has been bought from a paper mill, the tone of the writing will be markedly different than you would usually see from the student. Unless a paper mill essay has been specifically ordered by the student, it might contain sections that do not relate specifically to the topic in question.
If a student has turned in an essay on a topic, it is reasonable to expect them to be able to answer questions on it immediately after they have completed the work. Ask the student follow-up questions about particular parts of the paper, or about their sources. The student should also be able to produce research notes if they did not simply plagiarize the work from other sources.