Hi LOGU,
to measure the force of a motor you must measure the corrent absorbed from it.
You can use a shunt and an OPAMP to converte the current into a voltage readable from ADC of your microcontroller.
The current flow through the shunt is the same of motor current: this is proportional to the mechanical strength avialable on the shaft.
(encoder is used only to evaluate the speed and direction of rotation).
Best regards
