Let's make an experiment to test the presence of starch in leaves.

Experiment steps:

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.