Calculate torque from current induced in a single phase of a 3 phase AC Induction servo motor
The current of an induction motor increases with increased load, but the increase is not linear. An induction motor has a significant current with no load applied. Much of the no-load current is magnetizing current that lags the voltage by 90 degrees. At no-load, there is also a real (in-phase) component of the current due to motor losses.

Since the motor losses are small, the real component of the motor current can provide an estimate of motor torque, but the losses reduce the accuracy of the estimate.

I would expect there is a way to get torque information from the drive in the form of a data display or an analog output signal. I would expect that to be more accurate than any measurement that can easily be done by the user.

Slip speed is proportional to torque and the torque/slip rpm will be fairly constant over a wide range of operating speed and torque. Operating rpm can be measured with a tachometer. Synchronous RPM can be calculated from the frequency of the current. Slip RPM is synchronous RPM minus operating RPM.