[KevinH]

Yes use Sigil's built in Checkpointing and Diffing features. Load the epub and tell it to make no changes. Make a Checkpoint. Reload the epub and allow it to auto fix things. Then compare it against the Checkpoint using the Compare/Diff command of Checkpointing.

To prevent this cycle you need to change your workflow or you will need to get the author of modify epub to add the proper doctype to both the jacket and cover it creates. Or just let Sigil auto fix the missing doctypes and be done with it.