Is this triggered under certain workloads, or has it been chewing data for everyone, and how much?
Unless you are checksumming your files through a daemon run on a schedule (IDS style) AND manually review the results every now and then, you can't know.
Even if they have them, the diffing needs to be done.
Given the past problems with ZFS, I think I'll write a summarizer, that creates a database of metadata for each backup: it will be small to keep around, and may be use more easily to determine which backup contains the uncorrupted file.