The common definition of "wing incidence".The wing's true chord line runs from the aft most point at the trailing edge to the furthest forward point on the leading edge. For this "typical" "flat-bottom" airfoil (it isn't in reality), the true chord line is tilted up relative to the Fuselage Reference Line, or FRL. The angle is merely a mechanical setting to the wing relative to the arbitrary FRL, and is not the "Angle Of Attack" of the wing.

That AOA is a varying parameter depending on airspeed.

..Meterless incidence measuring.......

..click on the image for a 3x larger display... 1963x670..

..

Typical side view... set up the assembled airplane on a flat surface.. use a bubble level to get the surface flat.. Jack up the fuselage with the bubble level on top of the fixed horizontal.. or measure up from the flat surface and set the leading and trailing edges of the fixed horizontal to the same dimension..

... In the full-scale world, this is how it's done.. a pair of locations.. Fuselage butt-line, and water line will be specified in the TM to jack up the plane to level the fuselage reference line. (Note the T-28 sits with a tail-down attitude on the ground).. And the stabilizer is parallel to the FRL. The wing is positioned with positive incidence at the root, and washed out zero at the tip.. the motor has a 5 degree downthrust ..

..click on the image for a 2x larger display... 840x490..

As with the Cub then, leveling the horizontal suffices to establishing a level attitude from which to measure the other angles.. incidence, washout, downthrust... which will have buttlines, waterlines and wing stations to measure to in the TM.