Usually by the Reasonable Person Test: https://en.wikipedia.org/wiki/Reasonable_person.
It can be pretty subjective, but it often gets clarified with enough common law precedent (which can be amended with a new law if it becomes abused/unfair/odd).
It can be pretty subjective, but it often gets clarified with enough common law precedent (which can be amended with a new law if it becomes abused/unfair/odd).
The opposite is also interesting to observe: In common law countries the written law also becomes more and more fine grained limiting jurisdical law making.