Thanks - "ssh-add -L" talks to the ssh-agent and asks what keys are loaded. That shows all keys including keys from the yubikey and from the local filesystem.
I am looking for a command that shows what's on the yubikey.
From what I gather, if the command from the article is run: "ssh-keygen -t ed25519-sk -O resident", the key is stored in a FIDO2 slot.
If that's the case, my question is how to show what is in the FIDO2 slots and how to delete them?