My understanding of wing incidence is the upness for downness of the the wing as it sits on the fuse in relation to the centerline,not the twist, I'm building the 40 size cub clipped wing version andputting a magnum 70 4 stroke in it with working landing gear

What you can do when checking for twist is place a long something.. dowel... rod or anything else you know is straight on the flat bottom and see how it looks. The long rod, etc will accenuate any twist. This can be with the plane right side up or down.