[Al Skierkiewicz]

Jim,
The best way to mathematically analyze this would be a single step function. From what Greg has written, it would seem that the inductance of a winding is small. A scope would verify that the inductance likely does not have an effect until pulse width is very small. I just had a thought...it is always possible that IFI has built in a power curve to compensate for the average motor response at low speeds. They are pretty smart guys. As I alluded to before, the actual current through the motor changes depending on how many windings are in the circuit. Since there is no sync between the PWM frequency and the motor speed, you should be able to observe steps in the waveform when a motor is connected. Another method is looking at the voltage drop across a series resistor. This is a simple solution since the wire acts a resistor. Attach your scope probe to battery negative and scope the "-" terminal of the speed controller input. You will be looking at the motor current through the controller but that has little effect on your data. (again you are looking for representative data, not absolute.) You should see step currents here as well as inductive response. You may need to play with horizontal sweep to see the edge of the rising pulse. Calculate current from the voltage you measure. A 12" piece of #10 is .001 ohm, so ad a few feet of #10 between the controller and the battery. (If measuring on a real robot, 24' of #6 is .001 ohm so fudge the series resistance.)
Back EMF will not be measurable, just it's effects. Remember that the impedance between the motor and controller are very low compared to other reactances within the motor. If you are measuring anywhere after the controller, DO NOT connect the ground lead of your probe to the controller output terminal. It is not at circuit common and is only near common when the controller is in the forward mode. Expect large amounts of noise at the motor terminals from brush arcing.
Finally, a plot of PMW to actual average voltage (from scope measurements of voltage and duty cycle) would be a very accurate way to look at linearity with and without motor.