Armchair "engineering":
(my bad)One of these faults will be receiver drift and the test would be to send commands at different frequencies and see if there is a response.
Real engineering:
JPL does not send multiple frequencies, they actually calculate the shift based on probe temperature and some other parameters they observed during the pre-encounter phase.
How real engineering is done:
Even firing the maneuvering thrusters or switching on/off another instrument causes a known shift in the receiver frequency and a specific settling time.
As for the issue itself, JPL engineers modeled the issue locally and came to the conclusion that the fault is a defective capacitor in the receiver frequency tracking feedback loop. That matched the observed characteristics of the receiver and has allowed them to hit the receiver's center frequency essentially every time since the first encounters.
As I said ... there' no magic in it.