[...] how would I go about using the sense resistor if I wanted to?
It depends on what you want the signal to go to, but let's assume a small signal transistor (2N3904, BC547 etc.) and that the max. motor current should be 0.2A (just an example, plug in your own numbers).
To open, the transistor needs its basis to go around 0.65V (+/- ~50mV) positive, referred to its emitter (which should be at 0V.
To develop 0.65V at 0.2A, you need a resistor of: 0.65/0.2 = 3.25 Ohm
Nearest standard value is 3.3 Ohm. 0.65V / 3.3 Ohm = 0.197A (around 1.5% low, but much better than component tolerances).
The (open) collector can be read by a digital I/O pin with a pull-up resistor and the program will have to either shut down the motor or decrease the PWM duty cycle.
If you want to read the current by an A/D input, you could calculate the resistor for erg. 0.2V at max. current and amplify the signal with an op-amp (Av=25) for a 0 to 5V signal.
In each case, the resistor should be rated for the power it has to dissipate. In the first example it's 0.65V * 0.2A = 0.13W and in the second, 0.2V * 0.2A = 40mW - a 1/4W resistor will suffice in both cases, but you have to calculate this with your own numbers - if eg. the max current is 2A at 0.65V, you'll need a 1.3W or larger resistor, which means a 2W resistor minimum.