It's not a hard problem. Shipping companies must know the content of each container by law.

Verify 1 container out of 20. When you catch a stolen car, fine the shipping company for not doing their job. Find employees who performed the forgery of documents and put them in prison. If the company doesn't keep records of which employee prepares which document, fine the company. And so on. Unfortunately police and customs would have to do their job in this case, I can see how they're upset.