How did Matilda come to know the real cost of the necklace?

When Mr. Loisel had paid all the borrowed money, one day she met Madame Forestier. Although Madame Forestier could not recognize her, she was very amazed to see a great change in Matilda. Then Matilda told her the story of her life. She highlighted that she had led a difficult life for ten years so that she could repay the debt for buying the necklace. On hearing this, Madame Forestier was very shocked and informed her that the necklace was an artificial one and its cost was not worth over five hundred francs.