One (not uncommon) scenario is when two identical (or high match) assignments are turned in, and the professor does not know if (1) the students collaborated or (2) one student cheated off of the other. Penalizing both students is clearly wrong if one student did the assignment properly without cheating, and the other student copied the first student's work.

How can the professor determine which student cheated (or if both students cheated), without spending substantially time looking into it and talking to both students?