With regards to how you prove the negative, I make a best effort (usually via Google search) to find any local ordinances that might apply to the place I'm flying. For example, in Lexington,
MA there is a local restriction that prevents launching from athletic fields or from the "Battle Green". I generally don't fly over those areas even if I legally can from, say, the sidewalk, because I like to stay on good terms with my neighbors. That said, if someone challenges me where I have made a best effort to make sure I am legal I would take an approach similar to
@Pondhopper, with the assumption that it is the other person's responsibility to cite whatever rule they think I've missed. Though I also might ask them, in the interest of safety, to hold off the discussion until I have a change to land. But I'm not going to get into an argument, and would likely leave if the situation got heated.