What is the best way to measure the following DC motor parameters?
To measure back emf, depending on motor type, disconnect the wires (!) and spin the motor some other way (e.g., by hand, but with a steady hand), and monitor the voltage waveform of the disconnected wires on a scope. The scope will also make it really easy to determine motor speed. If it is a brushed motor, I think this may not be appropriate. The back EMF is a property of just the motor RPM and not much else, and the ratio of V/Hz should hold fairly constant, so if the manufacturer gave it to you, you should be ok as is. 

Another option to at least get an idea, and anyway useful for any design, is to monitor the voltage and current at the same time. You'll need some kind of current probe. A current sense transformer and a resistor can be a quick option, if not just construct a small value sense resistor from many 1 ohm resistors in parallel. Consider thermal dissipation. 

If you do calculations, keep in mind also that the applied voltage as seen by the motor, back EMF, and current, are not necessarily in phase with each other, so you can't just go plugging peak values of your AC measurements into formulas to get a complete understanding of what's happening.

Also, if this is important enough and you can spare a motor, consider removing the gears for the purposes of any measurements.