Have you got a couple 4' levels in the shop? maybe in different colours?

All winding sticks will tell you is...that there is a twist going on down the length of the panel...

Place a 4' level on it's edge with a bright light behind it....look between the surface and the bottom edge of the level...where the light shines through, THAT is the LOW spots...work your way from one end to the other end..and however you wish to mark any high spots, do so....wherever the level touches is the high spots...go until the bright light ( at the end of the tunnel) no longer shows through...