Is it possible to generate with rsync the report of the difference between the source folder and the destination folder? For example in DRY-mode generate info like:
Synced 631GB, out of synced 12GB of the data.
This sort of question should be asked on the mailing list, not by creating a bug.
As for your question, the only thing you can do to figure out actual content differences is use something like --only-write-batch.