I bought a cheap BLDC motor on some Chinese website and could not find its characteristics. However, I have A28-400 24V amp flow Dc motor with its characteristics. If I'm able to build a test rig to load the BLDC by connecting the shaft of BLDC motor to that of Dc motor and power up the BLDC using a commercial controller and measure current from DC motor (acting as a generator with a rheostat in series to vary load torque) and try to map values of current to shaft torque of Dc motor using its chart. How accurate and reliable is this method to measure torque assuming zero slippage at the coupling of the motor shafts? And I'll be loading BLDC motor till it stalls by gradually increasing its power and decreasing resistance on the other side.
My gut feel is you'll get it within 20% easily, 10% if you properly account for friction in the DC motor -- and without some independent way of verifying, you'll never know for sure.