What about online quizzes and discussion activities? How do we know another student isn’t taking the quiz, or if students are collaborating on quizzes, etc.? Or do we just let go of that and are satisfied with the knowledge that, if we use quizzes as a learning activity, that’s the point?
So long as we make a reasonable effort to prevent abuse of the grading system, I think that is the extent of our responsibility.
One suggestions is to use lots of low-points assignments that don’t have a “right answer” and the submissions are “transparent” – visible to the whole class (like discussions). With Moodle, these are easy to set up, grade and track. It is unlikely that a student can have someone else do a lot of the work – there is just too much with not enough at stake to make this viable.
Occasionally, I do get some submissions that are significantly different from the student’s “usual” work and that has to be dealt with on a case by case basis.