The person who seals it should be verifying the contents, or whoever declares on the paperwork should verify it. Again, in an organized crime situation, there are a lot of trusted insiders, so the person who's declaring could be a trusted insider. Unless you have multiple people in the chain who are testing that process, it could still slip through. That's the nature of smuggling for whatever it is that you're smuggling: It's trusted insiders who are the ones, quite often, responsible for falsifying documents.
Unless you're going to open every container.... It's the same as imports. We don't open every container. We don't scan every container that comes in. It's risk assessment-based. You would have to take a similar approach, a risk assessment-based approach.
The more you can do to validate the security credentials of the people involved in handling the cargo along the chain, the more likely you are to reduce the number of trusted insiders you have in the system.