retrolefty: I'm going to identify the motor terminals by negative and positive, so-called because they are negative and positive with reference to each other when the motor is being turned in the "forward" direction.
Connect a couple of 1k resistors in series from 5V to ground. 10uF capacitor from the center point to ground.
Connect the negative terminal of the generator to the center point of the resistors. That is at 2.5V.
Connect a 100 ohm resistor across the motor, from positive to negative. Connect a 0.1uF capacitor also across the motor positive to negative.
Connect the positive lead of the motor to a 1k resistor, the other end of the 10k resistor goes to a 0.1uF capacitor, other lead of the 0.1uF capacitor to ground. Connect the end of that 10k resistor to an Analog input pin on the Arduino.
Now when the motor does not spin, it is at 2.5V. Motor spins "forward", it adds to that voltage. Motor spins "backwards", it subtracts.
I might also connect shottky diodes across that Analog input pin such that they are forward biased if that voltage goes outside the bounds of ground or 5V.
Play with the values. This is all off-the-cuff.