Look for suspicious phrasing. Students usually write in a plain, utilitarian style and make errors in spelling and punctuation. Watch for unusually distinctive, stylish and error-free phrasing. If the prose sounds too professional, it may have come from another source. This is particularly likely if the suspect passages sound more elevated than the rest of the essay or writing the student has turned in previously.
Identify statistics and facts listed without a citation. Students lack personal expertise in many of the topics they write about. If an essay includes detailed medical, historical, scientific or other expert information, that information likely came from an unidentified outside source.
Check the dates on the essay's sources. Pay close attention to dated research. If the research is all from several years ago, that essay itself may be several years old and have come from a paper mill or another student. Also note the access dates a student lists for the websites cited. If the student claims to have accessed all the websites a couple months or more ago, he is likely self-plagiarizing: recycling an essay he wrote for another class.
Pick a sentence of questionable text and paste it into a search engine. If the wording was taken directly from an online article, the search engine can often locate that source. When you locate one article a student has plagiarized, skim through the rest of it. Very often a student will copy multiple phrases and sentences from the same article.
Use anti-plagiarism software. A variety of programs, many of them free, exist to help teachers and online writers locate examples of plagiarism. You can paste varying lengths of text into a program, and it will search the web for matching content.