Although I don't recall many cases of plagiarism, when I did suspect it I went through the homework with the pupil in question, asking pertinent questions, such as "where did you read that?", "why did you come to that conclusion?", "why did you decide not to mention ...?" and so on. Increasing embarassment and inability to answer very quickly led to a confession. If it was exam work, I took the same line as mrvonnegut - a dire warning of the potential penalties that an exam can impose if plagiarism is spotted.