My idea is to get a good, strong light source such as a spotlioght or high-powered flashlight, a long, level surface, maybe a drag strip or running track, and a wood or metal base that castand upright. A a closer distance, shine the light on the board and trace the outline of the circle of light. Then move the board back further and shine the light on it again. Measure how much further the light is both above and below the circle. If the light has extended an equal distance on all sides of the circle, then the light has not bent upwards. If the light has extended further above the circle than below the circle, then the light has bent upwards.
Just to clarify, the light source must be pointing parallel to the level surface and rhe board mist be perpendicular to it.