I hear in FL (Brevard county) they are quite picky about tint laws, and most sheriff have the tool available to check the "Visible light" percentage...
I'm guessing it's one of those things though, they aren't gonna just pull you over for a dark tint (oh hey, this car has dark tint, let's pull them over)...more of, "this guy is weaving between lanes, running off the road nearly twice, maybe we should check up...lol" - pull him over, find he's high off his ass, drunk, driving the vehicle intoxicated, so they also decide to check the window tint Visible light percentage while they are at it
Here in FL we don't have annual "vehicle inspections" but I've heard some states that do (more inspections, more money, more jobs, for people to inspect those cars? lol).
But yea, usually you can call up the local cop shop (or highway patrol, law enforcement, etc) and they will be able to tell you local law restrictions, or where to go to find it yourself.
But yea, I found a place here that tints that quoted me 100$ for the full car, 4 doors, back window, and the front window (above the AS-1 line only as per Florida law).
Figure I'm better off just paying something, but I'm kind of hesitant since I've heard so much stuff about the rear window/defrost lines, and I use my defrost quite a lot.