I have an idea using two laser pointers, beamers etc. and small mirrors mounted temporarily to the control surfaces. Mounting model vertically next to a wall few feets off so you can use tape measurer or even draw some sort of scale to the wall. For lasers there is a need for a simple wall mounting device which you can adjust height so that when neutral they are aligned horizontally with reflected laser dots which makes scale equal to both directions.

Because of reflection the scale will be logarithmic in the wall and multiplied with two and it limits the control surface deflection you can measure but with aerobatic models I believe it can be done in normal workshop. Usually control surfaces taper so flat mirrors won´t be aligned with zero line. Therefore model does not have to be perfectly vertical but needs some tilting. This principle should be really accurate as long the beams are horizontal when neutral.