How do I report a fraudulent foreign adoption and remain anonymous?

Call the adoption agency that the child went through, or the local department for child welfare or something.

How do you know it's a fraudulent adoption? BE very careful, if USCIS investigates it could mess up these people's lives... and if there is no fraud they could sue you for damages, loss of time at work etc.

You can't be anonymous when giving "tips" like this, at least not with immigration authorities.