As a TA, I'm well aware that the students getting As are mostly collaborating and that the "lone wolf" students tend to do worse despite putting more effort in and possibly having a better understanding of the material. And I feel bad about it. But aside from horrible spyware, I don't see any good solution that would make exams more fair for the "lone wolf" students. My approach is to grade in a way that's disproportionately harsh when I see multiple similar-but-wrong answers and lenient when I see a unique wrong answer.