Dr Chris Willmott of Leicester university said teachers need only to type five or six words from the student's work on to a web search engine to check it.
The tip comes amid continuing concern over students using the internet to cheat at coursework. Several sites allow youngsters to download entire essays, although teachers have said that cheating is fairly easy to spot.
Dr Willmott, a biochemistry lecturer, devised a plan four years ago to help spot plagiarism after he caught two of his students lifting material from the same source. Since then he has only had two incidents of plagiarism - both this year.
GCSE and A-level teachers should also look out for unnecessary changes in font or formatting, American spellings and references from obscure or mainly overseas publications, he said.
Dr Willmott suggested teachers could specify books or websites to be used for research, or even get youngsters to make an oral presentation before handing in coursework.
He said some examples of plagiarism could be unintentional. Sometimes, after taking notes from books, students would return to them and mistake them for their own work.