Let's make an experiment to test the presence of starch in leaves.
1. First let's have two healthy plants which are potted. Remember that they should be same type.
2. Give numbers to your potted plants.
3. Keep number 1 potted plant in dark room for couple days. Because keeping plant in dark room will remove all the starch from the leaves.
4. Keep number 2 potted plant in sunlight.
5. After couple days take one leave from each of plants which are numerized. Then put a few drops of iodine solution on leaves.
6. Please note it what is going to be change in the colour of the leaves.
Your observation will be:
You will not observed blue-black colour on the leave of plant number 1 kept in dark room. This mean it indicates the absence of starch. You will observed blue-black colour on the leaves of the number 2 plant kept in sunlight. This mean it indicates the presence of starch.
Plants prepare glucose during photosynthesis which gets stored in plants in the form of starch. Iodine turns blue-black when reacted with starch.