Use a strain gauge. Easiest is to clamp a beam to
your shaft, and restrain the end of the beam with a
rod in tension.

Epoxy a strain gauge to the rod, and away you go.

Annie

or, if you want something easier to deal with electronicly
but a bit messier mechanically (and if you can cope with
the rotation)-
attach a rotary pot to the shaft and restrain the shaft with a
torsion spring.

guys, when you're looking for sensors, ya gotta have some
idea

a) how much input to expect (the scale)

and

b) how much the sensor can affect the object (as here - how rigid
do I need my torque sensor to be?)