how can you identify that the part in which food is stored is stem?

Answer 1

Food is stored as starch. Starch turns dark blue when it contacts iodine.

So apply some iodine and see which part turns dark blue.

Microscopically, the food storage cells are generally large, round, and puffy-looking.

Answer 2

Look for xylem and phloem, the "veins" that the plant uses to transport food and water. A good example of this is celery: bite into a stalk of celery, the stringy bits that get stuck in your teeth are xylem.