Testing to see if a shaft is turning

Hi, I wonder if someone can offer a newbie some guidance here - I have a small DC motor connected to a shaft which has a black/white encoder disk and a simple IR emitter/detector - I want to use the signal from the IR pair to stop the motor if the shaft stops turning. I've got the wiring sorted and have tried playing with the pulsein command and can see the signal but I can't think of the simplest way to link this to the motor kill? Any help appreciated!

Just check a timer periodically, if you haven't seen a disk transition in the period, trigger the kill.

Measure the voltage applied to the motor (analogRead() - resistor divider etc) .

OK you are not sure it is turning but it should be :)