This is what I managed to find:
Tests that do not require euthanasia can be used to examine serum, spinal fluid, and saliva, but none of them can rule out rabies with 100% certainty.
Here's an article on the saliva test:
Note: Detection of Rabies Virus Antigen in Dog Saliva Using a Latex Agglutination Test
I don't know of you can get this test? Or how effective it really is...
