As ecerfoglio and Boncuk have suggested, it may not be a good idea to completely stall a 3 phase AC motor, at full rated current and voltage.
My thought was that a plot of several data points, for different values of external load torque, could provide some useful information regarding the objective. .... Employing graphical extrapolation, rather than a physical destructive test. ... This is only speculation, and may not be practical.
The method suggested by ecerfoglio seems to be the most promising ... and is unlikely to damage the motor.
You may want to verify that the three phase currents are all the same, or approximately so, when you take measurements. .... This is not always the case.