How is speed torque curve of an induction motor plotted experimentally?
Usually in college laboratory they connect brake drum and spring balance meters with motor's pulley  to measure the force. (you may refer this image below i attached)

While running test you need to measure the speed using tachometer and spring balance force.

And need to find the radius of brake drum .

Torque= force*distance 

Torque= (difference between spring balance )* (radius of brake drum)

In industires they may use test benches they can easily do this