The incidence is measured near where the wing meets the side of the fuse.

The attached is an image of a SIGLT-40. Ihope that it conveys the idea. With the tail supported so that the stab is level, you can measure the LEand TEof the wing at the fuse. The aparatus shown on top of the wing in an incidence meter. It has a very accurate swing pointer and scale. There are digital meters that replace the swing pointer with a digital display.