I just gave a suggestion on how to control this matter
here
Also my wife suggested we just require everyone to put all spares in your shipping container. Then the only thing allowed into pits is raw materials and standard products. No assemblies. But shipping would be a little more.
Anyhow, I don't see the need for this whole discussion unless your dealing with a flagrant in your face violator. Which I just can't fathom. I think it makes a lot more sense to put a system in place to check the wrongs before they happen. It's in place in every other aspect except this spare parts rule. So I think it's actually a simple answer. Provide the check and balance. And as far as the scale of the effort, I think it could be manageable if made a spot check system. Like Aerfin said, you can't prove anything unless you've got a system in place. It's not good to have a tattle tail system instead.