Hold a hand between the source of light and the spot where a plant is to be set. The amount of shadow gives a rough measure of available light. If there is no shadow, or if it is very hard to see, low light exists. If the shadow is somewhat blurred, but definitely there, medium light exists. A sharp and distinct shadow indicates high light conditions.
Plants lacking sufficient light exhibit several symptoms.
They will be taller and more spindly than normal and may have smaller, paler leaves. The spacing between leaves will be longer than normal. Flowering plants will fail to bloom normally, or may develop flower buds then drop them. Leaf drop may occur, although leaf drop alone may be a symptom of many things. Plants having variegated leaves may produce entirely green leaves.