From what I know, credit rating agencies are allowed access to a host of "confidential" court material which the public are totally barred from seeing - they simply buy it directly from the courts (you and I cannot). Credit rating agencies can see pretty much everything held by both private/commercial and public bodies that you might regard as personal information. I would not be surprised if the CSA can access even more, but I do not know for certain.
I don't know the menchanism but if they couldn't validate the info in this way I expect there would be huge problems with fathers undertstating their true earnings