It depends on the law really. There is no one rule.
For example, owning lockpicks is in many places not illegal, but owning lockpicks with the intent of bypassing a lock is.
Some laws are very specific about the severity or testability of a crime where as others are not. In that case a judge has to interpret the criteria for legal tests, either from previous case law or by building new case law.
In any case, being charged for something or not is a completely separate issue. Things are no less illegal just because the state has no resource or will to execute the law.
Also, being charged does not mean you broke the law either. Nor does judgment determine it (although it's a very strong hint) since a latter appeal could acquit you of chargers.
The determination of guilt is in the facts of what happened. And that's the whole point of the legal system. Being charged, getting judgement, appealing. It's all a process to determine guilt or not. It is not itself the mechanism of guilt.
The idea of a "guilty conscience" enshrines this idea in expression.