Sensing gear rotation

I am currently working on a project where a 12V DC motor will be used to rotate a reel (one used for fishing).
I would like the motor to turn on once a given force is detected (this force will come from the rotation of the reel).
The motor will be geared, so I was thinking it might be possible to use a photo interrupter, such as SEN-09299, to detect rotational speed of the pinion gear given the number of teeth that pass through the photo interrupter in a period of time. Then I would define a specific speed (or number of teeth passing though per second) to turn on the motor.

Would this be possible? If so, how would I start my code for this?
If not, what other options are worth exploring? I considered a force sensitive resistor such as SEN-09375, but I don't think it will be possible.

If the gear is made of a magnetic material such as steel (but not stainless steel) then a "biased Hall Effect" sensor can be used to count the teeth as they pass by. This is a Hall effect sensor with a magnet behind it, and may be easier to mount and align than a photo-interrupter.

Will the expected force on the line be sufficient to cause the gears and motor to rotate? If so then the motor will act as a generator and that would be easy to detect.