As per the WebAuthN spec for Privacy, the authenticator shouldn't disclose the availability of keys on the device(although as a trusted RP knowing this info will be greatly help optimizing UI for end user) https://w3c.github.io/webauthn/#sctn-assertion-privacy
But seems like I can guess the availability of keys by doing an assertion request with this option https://developer.apple.com/documentation/authenticationservices/asauthorizationcontroller/requestoptions/3994648-preferimmediatelyavailablecreden
Although the error code is generic, based on timing information like if it returned immediately instead of opening the QR code fallback, I can guess the availability of a key.