Hello,
1.- You can set up a electromechanical model of your motor. Get L, R and Kv (speed constant) an feed it with your power pulses.
2.- You can measure your motor's Kv and remember that *IN COHERENT UNITS* Kv is aproximately numericaly equal to Ki (torque constant) wich is, usualy much difficult to measure.
Sorry, no papers....
Maybe using Google you can get something usefull.
I've done it some 20 years ago using SPICE and CSMP running in a IBM370.
Any decent control systems book will give you a mathematical model for a DC motor.
best regards
Another round about method would be to use the efficiency of the motor. Mecanical power out = efficiency x electrical power in. The efficiency should be one of the rated parameters from the data sheet.