This varies between schools. At my current school it's fairly straightforward to say "I saw this student looking at X's paper" and show that the answers are mo

This varies between schools. At my current school it's fairly straightforward to say "I saw this student looking at X's paper" and show that the answers are more similar than would be expected. Multiply choice would be harder of course, but free response or incorrect math calculations are pretty convincing. Students have recourse, but the appeals process here is not much work for the faculty member making the accusation until maybe the final level.