Ok, say you have a "National" Long Duration (type D) visa for a Schengen country. Let's say it's Italy for argument's sake. Let's also say that you are also from a country whose nationals don't need a visa for short stays in other Schengen countries (example, the US, Canada, etc).

Now let's say that you have been in Italy for over 90 days, with your Long Duration visa, and decide to visit France, and while you are in France you are stopped by the police and they ask to see your passport, which shows that you have been in the Schengen zone for over 90 days. Would that present a problem for you since the visa you have is a National Visa, and you have no way of proving when you entered France? What would happen to you in that case, would the French police officer consider you are in France illegally?

I'm just curious how "national" visas can work in an area where there are no borders!