Or you can estimate motor Kv from datasheet - many suppliers even publish the nominal value. There are many variants of the RS-365. For example:
I assume this is an O-gauge engine motor application. There are several issues to address with the external resistor sensing method. With AC track voltage, the DC source voltage will not be stable like from a battery - or maybe you're talking about battery operation(?). The rectified motor source voltage will fluctuate several volts in sync with 60 Hz. The common use of hi-frequency (kilohertz) PWM pulsed motor drive can also confound voltage measurements. So I suppose you could make a differential voltage measurement across your sense resistor - but that has issues of its own. Then there is the fact that with a DC can motor like the RS-365, the back-emf voltage itself fluctuates as the motor commutates between the windings/poles - typically 3 or 5 per revolution. So the measured back-emf voltage also fluctuates like a rectified sine wave but proportional to RPM rather than 60 Hz. And so on. All of these issues are well documented in DIY electronics forums.
But to the original topic, I don't think it's practical to use the back-emf speed sensing method to achieve the level of accuracy/precision/repeatability required for DCS applications in general or to MUs specifically. Yes, the HO/DCC guys have all kinds of programming methods to trim/calibrate back-emf engines for mix-and-match MU operation...though it tends to be a your-mileage-may-vary especially when Mu'ing different engine types.