The best way to assert whether a law is producing more benefits than not is to define sensible metrics for the areas or subjects it impacts or affects.
It is in fact a problem of hard resolution until addressed with precision, objectivity and correct data.
Probably the best way to tackle this problem is by properly identifying the problem that a new proposal is trying to address. In the proposal, all primary and secondary aspects altered by the proposal should be enumerated and taken into the account of the overall economy of the suggested solution, in comparison to the status quo.
When all the parts of the problem have been identified, these will also constitute the metrics we need to inspect and monitor in order to objectively or at least clearly make a stateement about the results obtained by the introduction of the new rule. In theory this process should really lead to no ambiguity: the law was right or wrong because of sound, measurable evidence.
A system of rules should encourage the definition of strong enough ‘audit metrics’ and the process of acquiring and analyzing the resulting data should be part of the core featuress of an implementation of the overall concepts.