O2 SENSORS

How do I know if my O2 sensor has failed?
The O2 sensor is set to 20.9 during start-up. An expired sensor will possibly show all dashes "- - - -" instead of 20.9. Other indications are that the value will initially be 20.9, but then increase to a higer value such as 25%. Rapid changes of large amounts are also an indication such as dropping from 20.9 to 15%.