You need to know if a panel is in shadow or not using this 2 vectors and using cosine law
panel verctor->
v=Sketchup.active_model.selection[0].normal
sun vector->
s=Sketchup.active_model.shadow_info["SunDirection"]
But you need to know also if there is some object projecting shadow over the panel using raytest with sun vector and some points of the panel. You can modify this step and look for other methods if you need more accuracy...