This board is able to detect a stalled fan and set an output to trigger external actions.

Overview

This board uses the MAX6684 IC and some support parts to detect the current flowing through the fan and sets outputs based on disruptions in that current flow. By varying the value of capacitor C3 the system can be tuned for different fans. "Because of the complexity of fan-current waveforms, the value of CF has to be arrived at empirically and must be verified by bench testing." From the datasheet. The datasheet recommends 0.1uF as the starting point

More info can be found here: Fan Failure Detection Board