Context change within a student's paper. For example, a student quotes a statistics from Great Britain, and the sources in the bibliography are published in the United States. Plagiarizing students, if confronted with missing footnotes, may point to a bibliography and claim ignorance of the need for footnoting. When this happens, an instructor could schedule time to discuss the issues in the paper or in the sources in the bibliography. If the student doesn't have a working knowledge of the subject matter, he or she probably didn't write the paper.
Instructors can check to see if the sources listed in the student's bibliography are real.
One hint might be whether or not the library holds or subscribes to the titles. Another hint might be whether or not the book is out of print. Students who have not cheated deserve fairness. Some paper mills have been in existence for 30 years. Paper mills have printed and sometimes searchable online catalogs to the papers they offer.