You might set up a conference time with the student and ask him or her to bring complete sources and rough drafts so that together you can compare drafts and the use of sources.
Harris suggests asking specific questions about the content of the paper ("What do you mean here by the phrase...?" or "In what sense are you using the word...?"). He also provides examples of questions that you can ask about sources during a conference:
Tell me how you researched and wrote this paper. What process did you use?
Where did you look for your sources? Which libraries or databases did you consult?
Where did you find this article...? ...Can you bring me a copy at the next meeting? (Harris 97)
Lastly, Harris suggests asking specific questions about the context of a quotation or a student's "overall opinion" of a book (97). Whichever method of questioning you choose, a non-aggressive approach and allowing room for the student to tell the truth will help make the conference most successful on all counts.