Your ears will tell you if the RRPM is right.
In as many words as you wish to use....define "RIGHT" as used in that statement will. you?
If either the Engine Tach OR the Rotor Tach fails.....that is one thing.
If both fail....that is an altogether different situation
If the aircraft is properly rigged....in autorotation with the Collective fully down....the Rotor system should maintain sufficient RPM to achieve a safe descent.
An overspeed of Rotor RPM in autorotation would be a far less serious event as compared to too low an RPM in autorotation.
So how does one Ear....accurately determine the Rotor RPM without the assistance of a Rotor RPM Indicator?