I'm told the relation between PWM output and the motor's rpm should be linear once the motors start running, meaning if the motors start running at 1200 pwm then at 1600 pwm they should be at 50% of their max rpm (2000 is the max pwm output). Don't take my word on these values (except the 2000 one) as they are for the sake of explanation and I don't know them by heart.
However this is in theory, by my experience on this I can tell you that this won't be the case most times, as slight differences on the motors/ESCs will make it impossible to get an exact rpm value. You should be able to get an approximation, but you'll have to account for many other values (propeller resistance, copter weight...).
Meaning getting the values the copter is outputting to the motors is simple, but measuring their rpm without physically doing so will always be nothing more than an estimation.