Identifying the common online source of mathematical work Students A and B in my physics class turned in identical solutions to certain homework problems on two problem sets in a row. My policy is not to give credit in situations like this. The similarities are far too extensive and idiosyncratic to be explainable if the students had really done their work independently. Normally this kind of thing is not a big deal, because the assignments are only worth token amounts of credit. The students get the message and change their behavior.

But this particular case is becoming more of a big deal because A acts very upset and claims he did the work totally independently. I asked B what happened, and B said he found a solution online and copied it down. This seems plausible: A and B both copied the same online solution.

How could I determine/prove whether this is what happened? I have heard that there are web sites that students can use to access materials like these. Is there a small enough number of such sites that I could check each one to see if this work originated from materials provided there? To gain access to these sites, would I have to upload materials that they wanted, which could be onerous or unethical? If this was an English paper, I could google for the plagiarized text or use a service such as turnitin. There are also tools for use with computer code. But AFAIK there is no way to do this with mathematical material.

Please do not post comments or answers about whether it is a good idea to count homework in students' grades, or whether I can really tell that two students' work is too similar to be explainable if the work was independent. These could be worthwhile things to discuss, but they are not the subject of this question.
From a comment:


  The solutions the students turned in are basically just equations with
  no surrounding explanatory text.


Ask student A to produce the explanatory text for the equations in the common answer. If A indeed worked out the answer independently, A should be able to explain the reasoning leading to each equation. If A just copied down a solution that only contained the equations, that is less likely.