An incidence meter is reasonably accurate and gives numbers to look at.

The most accurate way to check relative incidences along a wing is to attach 1/8" dowels about 24" long with a rubber band to the bottom surface of the wing so that several inches projects ahead of the wing. The rod has to be straight, I use carbon fiber rods. The rod attached to the wing root is the "zero reference". Stand back several feet and sight spanwise towards the root. Another rod placed at mid-semi-span should be parallel to the root rod (indicating identical incidence), and a rod placed at the wingtip should have it's tip lower than the root rod, indicating less incidence, or washout. The exact amount can be calculated by using trig.
For a built-up wing, twist and shrink the covering to make corrections. For a sheeted foam wing, a error is a warranty claim....