SNPtheCat said:
Let's say you leave the handling to a third party. How would one determine who is cheating, and what standard would one party have to reach in order to show that the other party has cheated? If you set the standard too high or too low, it either becomes ineffective or open to abuse, or both.

The core principle would have to be that the couple themselves, not the third party, would define what constitutes cheating. The third party would simply be the impartial administrator of those pre-established rules.