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.

Unfortunately quarantine and observation isn’t effective as a testing strategy for animals, the disease can progress at different rates and it’s possible for infected animals to show no symptoms. As rabies can’t be treated after symptoms start showing, they can’t take the time to do this.