Oh hai dere.
It sounds like you're primarily worried about the age of the girls and if the porn is underaged or not. That part is pretty easy to solve. Every legal site is required to have a
2257 statement where the government or concerned agencies can request documentation to verify that the model is of a legal age.
Here's an example of one (amazingly this link is SFW).
If you look around the site, and there is no 2257 statement, its pretty likely you are in worrisome territory.
If its less about age, and more about what KIND of porn it is, like if animals or other things are involved, then the laws vary quite heavily by state and country, and additionally credit card processing organizations have specific rules enforced upon them by Visa, Mastercard, etc. A good example of those kinds of rules are found
here, particularly the part titled "3. Violation of CCBill Policy." If you want to be ultra safe, make sure whatever site you are on does not break any of those rules.
Note this is the ultra paranoid way of going about things, as you can see things that can not be processed for includes things like fake celebrity sites, which obviously you won't break any laws by viewing, but it also includes things like rape, incest, and bloodletting sites which can be against the law in various locations.
Also, note that most tube sites contain content they did not pay for, so in that respect, most of them are breaking the law by default. However, whether or not you would be held responsible for watching things on those sites are about the same chance you would have of being nailed for watching a copyright infringing clip on YouTube.
(This may be the only time in my life I really felt like typing /thread at the end of a post :P)
Last edited by RayPowers; 07-22-2011 at 10:58 AM.
Reason: (added last tube paragraph)