The census code might be useful as an illustration but I think it would be better to write a new tool for the purpose. If the apt repository is managed by reprepro, you can use the checkpool command.
The main reason I want this is checking our historical snapshots are intact. Unfortunately while these were created by reprepro the reprepro databases were not saved as part of the snapshot. So using checkpool is not an option. I'm currently hacking together a quick and dirty python script.