Torque is given by:
$$
\tau = \frac{P_{shaft}}{\omega}
$$
Where \$P_{shaft}\$ is the output power of the motor (shaft power) and \$\omega\$ is the angular frequency of rotation.
The power supply in my country is 50 Hz, so a 4-pole motor has a synchronous (zero slip) speed of 1,500 RPM. Converting to angular frequency (radians per second), we get
$$ 1,500\ RPM \times \frac{2\pi\ rad.s^{-1}}{60\ RPM} = 157.1\ rad.s^{-1} $$
Slip is 2%, so the motor is running at 98% of synchronous speed :
$$
\omega = 157.1\ rad.s^{-1} \times 0.98 = 153.9\ rad.s^{-1}
$$
The output power of your motor is 100 HP. In metric units that is
$$
P_{shaft} = 100\ HP \times \frac{746\ W}{1\ HP} = 74,600\ W
$$
Giving
$$
\tau = \frac{74,600\ W}{153.9\ rad.s^{-1}} = 484.6\ N.m
$$
Note I have written down the units of every figure I have used. This makes it much easier to spot mistakes.
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.
Best Answer
slip per torque is proportional to magnetic field. so a weaker field will cause more slip. (I'm not speaking accurately it might be a square relationship etc.)
Most motors are already operated with the stator near magnetic saturation (else there would be excess iron in the stator) so it's not practical to increase the magnetic field.
one approach that may pay off is to use a different type of motor - the permanent magnet types do not slip in normal operation.
another approach would be to use a larger motor or to add another motor possibly coupling to back shaft of the existing motor.
another would be the replace aluminium in the rotor with a better conductor like copper or silver. As Nail Tosun comments cooling the rotor also reduces its resistance and thus reduces slippage a little.