Howdy Robin,
Unfortunately, two-wire fans are usually used for open-loop speed control and don't provide a TACH output pulse. As a result, there is no easy way to determine the state of the fan. The fans only have two leads, one of which connects to ground. The other lead is used to control the fan speed by adjusting the input (dc) voltage, or through a PWM signal.
Three or four-wire fans can be used for closed-loop speed control, as these fans provide a TACH output pulse that indicates fan speed, and would trigger the AMC6821 alarm if any fan faults are detected.
Best Regards,
Matt