The issue with that is that the diff command will download the full file contents from the repo, which is in many cases expensive. The solution offered by @alexweiss seems like the only available option that does not include downloading everything from repo. Essentially:
restic backup --force
restic diff -v <known_good_snapshot> <last_snapshot>
Note the -v to get a list of all new/modified/deleted files.
For some more thoughts on this, here is another thread discussing a similar problem, where the idea of getting checksums for files in the repo was also discussed. That turns out not to be possible without full downloads (with the current repo format at least). So the backup-then-diff solution is the best.