Often, searching the web using a key word or phrase in a search engine such as Google will produce a source. Or, the paper may have misleading references, which can also provide useful information. Finally, some hours may have to be spent checking the relevant bibliography and reading through possible sources. At this point, consulting with the Office of Undergraduate Studies may be very useful. The department maintains a list of former offenders, and the Office of Undergraduate Studies is likely to know what kinds of plagiarism are common in any given year.
Individual instructors involve the student in a discussion of plagiarism at different points.
Some prefer to mark the paper including a penalty for the plagiarism and warning the student that the consequences of this kind of dishonesty can be much more severe. Others return the paper asking for a rewritten version and giving a maximum possible mark for the assignment. Others simply research the plagiarism and advise the student as to the penalty. The department believes that plagiarism penalties must be closely linked to the circumstances of each case, and thus are the province of the individual instructor. Often a meeting between the student and the instructor is useful.