A Plagiarist’s Tale offers many good ideas on how and why we are dealing with plagiarism.
I once got a paper I was certain had been plagiarized. Like you, I couldn’t find the source. At the next class meeting (this was a class of only 20), I asked everyone to write down the thesis and basic argument of their paper.
The idea was that I would find out whether the student in question really, truly, wrote brilliantly or whether – as I suspected – he had stolen those words. Lo and behold, his off-the-cuff, handwritten paragraph was every bit as perfect as the paper, in different words. So there you go.
But it would have provided a useful piece of paper, had I needed to confront him. I had visions of saying, “So how come your written work is so ungrammatical, sloppy, and simplistic here, yet so sophisticated and polished there?” and watching him squirm. It didn’t work out that way, but perhaps it will for you?
I know some professors who’ve had the students come into their office and then asked them to discuss the work they turned in, without giving them a copy. Sometimes if it was a purchased paper the students haven’t even read the work.