A gas turbine engine includes a fan and a compressor. A combustor drives a turbine, including a first turbine with a shaft to drive the compressor. A fan drive turbine drives the fan through a speed reduction. A sensor senses a speed of rotation of the fan and communicates sensed speed information to a control. The control develops an expected speed for the fan. A problem is identified should the sensed speed be less than the expected speed by more than a predetermined amount. A method is also described.