Sadly the only way to know for sure is to test brain tissue, including in people, so there’s no way to be certain without killing the potentially infected animal/person.
There are a bunch of tests they can do for people that can potentially identify rabies (lumbar puncture, CT and MRI scans, skin biopsy, antibody test) but apparently it can still be easily missed even with all of these tests so they aren’t enough for an accurate diagnoses.
Masque of the Red death is a Edgar Allen Poe story not De Sade. If De Sade had written it the protagonist would have tried to fuck death.