To answer that, someone knowledgeable about the issue compares the facts at hand with the wording of the statute and the interpretations that the courts have provided over the years of how that wording applied to various similar facts.
In most cases, the answer is straightforward - often clearly reported in publicly accessible material. In some quite complex and highly nuanced cases, a deep understanding of the court cases will be necessary. And in the most difficult situations, a court will need to decide.
The need for the interpretations arise because circumstances evolve for a myriad of reasons - the common law has a built in ability to adapt to the changes of time.
In the vast majority of cases, the answer is quickly and easily determined by looking up the issue on the sort of materials referenced by Duceman.
Based on what I know about what Alan does with his photography, I'm confident that Alan will get any answers that he needs from those.