Miley Jab Hum Tum S04 E17 Mayank Misplaces Nupur's Gift

Mayank misplaces the locket Nupur gifted him and begins to panic. He soon finds out that Chinu has the same locket. After a lot of begging, pleading and negotiating, Chinu agrees to give the locket to Mayank. However, Nupur refuses to believe that it's the one she gifted him. How does she know?