[In the case of grammatical errors,] I was alerted to plagiarism by the sudden appearance, in a paper that is otherwise a morass of grammatical errors, of a series of flawless sentences with complicated structures. The correct use of a semicolon is a big red flag for me. As is the use—and often misuse—of specialized jargon or technical language that I’ve not discussed with them in class. Then I type those sentences into Google, and they all wind up being smoking-gun cases of plagiarism. My favorite case this semester was plagiarism within plagiarism. When I informed this student that I suspected her paper was plagiarized, she said to me, “I got my paper from one of the students who was in your class last semester. How was I to know that she had plagiarized?” Which indicated to me, along with a number of the other email responses I got from students, that many of them don’t even know what plagiarism is.
A fascinating dialogue between a professor who's caught students plagiarizing and a person who gets paid to write papers for students (via Maud):