How would you test the presence of starch in leaves?

Take a leaf from the plant, decolourize it with boiling alcohol, and test the presence of food (starch) with iodine solution. The food prepared by plants (carbohydrates) through the process of photosynthesis is stored as starch. This starch reacts with the iodine solution to change to blue-black colour. Only those portions of the leaf that contain starch would change to blue-black colour when tested with iodine.