Well, there are force sensors on each column. Between the column itself and the Control cables. If one pilot pushes on the column and the other pushes you will see this as positive and negative values. No need to activate the breakout mechanism to see conflicting inputs.
We are now talking about the force exerted on each column, as you say the column deflection will be the same (who is strongest) until the breakout mechanism is activated.
And yes, the sensors are in the AMM.