How is pornography determined to be underage?

Let's say a website is advertised as being underage pornography, is targeted as such by the police, and is visited as such by its users. The cops crack down on the site, then the users, afterwards discovering that all of the pornography was completely legal. Do they instead get the owner for arbitrary legal crap like age book-keeping etc, and what about the users?

Even more tricky- what about homebrew porn? How do they determine where that came from, who it is, how old they are?

False advertising, maybe

New laws require very strict records to be kept about the age of the models and such. I don't know how they did it before, but know it will be for arbitrary legal crap.

hnnn

Pedo thread. Now nice.