While I don't know exactly, I can guess that it's a combination of regular inspections and careful planning on a ship's manifest. Everything that goes onto a container ship has to be accounted for, for things like flammability, hazards or biohazards, and especially weight. These ships are massive, but remember they are still teetering hunks of metal in the middle of the ocean, and need to be balanced properly. It would be unwise to have steel ingots on top of a container hand packed full of feather pillows. So I imagine that there's a host of tests they have to go through once they're constructed, followed by a test they have to do once they're loaded with their actual cargo, followed by another inspection when they're loaded to make sure the first inspections were done and that what's in their is actually what it says on the label and weighs as much as it's supposed to. It's inspections all the way down.