Look up your digital Release in the database, then step back up to the Release Group. And hunt from there. Some releases will have full booklets attached in JPEG\PNG format.
I don’t know of a way to automate this. There are a few Userscripts that will add a number to the Covers page on the Release showing how much art is there, but don’t know if one exists that would show you these numbers at Release Group level.
What you really want is these numbers appear on the Release Group page and it would speed up your search.
Something like this userscript at least lets you skip all those with zero art. (And makes everything look prettier) Display cover arts in MusicBrainz pages: “mb. FUNKEY ILLUSTRATED RECORDS”