The justice system is designed to handle these ambiguities, it's literally the job of the system to gather the facts and experts and make a decision. If it gets to the point where you can't tell at all out seems like you would just need a new law that requires you to prove it was in public or get consent (which you should get anyway, even in public). Proving it was in public would be trivial for the case of something like a protest (lots of eye witnesses and security cameras). I guess I'm not seeing the scenario where it will ever be that unsolvable of a problem in society.