To date, there are no tests available to diagnose human rabies before the onset of symptoms. As such, treatment will be started presumptively if a person has been bitten by a wild animal or any animal suspected of having rabies. Given the deadly nature of a rabies infection, there is really no reason to wait.
What that being said, if the suspected animal is dead, tests can be performed to confirm the infection by taking tissue samples from the brain. If there is an asymptomatic animal, animal control will put it down so that brain tissue can be extracted for testing.