I can only speak from my experience in MD, and I'm sure it works differently from state to state, but there a cop needs no "proof" whatsoever to write you what they call a "repair order", which is what you would get for improper tint on your vehicle. In fact, the cop can write you a ticket for whatever he wants, and then the burden is on you to have your vehicle reinspected to satisfy the conditions of the order. In some cases you need to go to an authorized inspection station and in others, such as with tint, you need to be inspected by the state police at the MVA headquarters. In fact, the cop can be completely ignorant of the law and write you a ticket just because he "thinks" you're wrong. In my case, I was written a ticket for improper tint on the back window of a pickup truck. The law puts no limitation on any tint behind the driver and passenger, so having limo black tint from the passenger windows back is 100% legal (or at least that's how it was 10 years ago when I got my ticket). So because one cop was mistaken , about how the law was written, it was my responsibility to prove my innocence by taking a half day off work to drive up to the MVA headquarters and get reinspected.

As far as factory tinting goes... Perhaps it was different from year to year? I know that my stock '99 windshield had some sort of tint or filtering from the day I bought it. It's not apparent when looked at with the naked eye, but whenever I wear sunglasses there is a distinct greenish-purple "checkerboard"-like pattern across the whole windshield.