Locate the light source
Look at shadow direction and the bright areas on a surface. Infer where the light is coming from, then test the idea by moving the subject. The relationship becomes clearer through a small physical change.
Compare soft and hard edges
Observe the transition between light and shadow. A broad source and a small source can give the same subject very different texture and volume. Choose the effect that supports the picture's intention.
Watch mixed colour
Window light and indoor lamps may create different colour areas. Decide whether that contrast contributes to the image or distracts from it. Keep an unedited version when testing corrections.