I am trying to display a manifest from another IIIF server in my local viewer, but it doesn't seem to be working. How do I confirm the manifest is valid/not broken?
There's a validator on the IIIF website that will validate IIIF Manifests, at http://iiif.io/api/presentation/validator/service/.
If you've got a lot of manifests to test, you can also download the validator itself at http://github.com/IIIF/metadata-api/.