What foreshadowing there is in "The Necklace" is very subtle because Maupassant has intended that the narrative concludes with a startling ironic reversal.
After receiving the invitation to the reception at the Ministerial Mansion, Mathilde Loisel visits an old friend to ask her if she may borrow some jewelry to wear with her new gown. Among the jewelry her friend shows her, she finds a necklace that she likes.
All at once she found, in a black satin box, a superb diamond necklace; and her pulse beat faster with longing. Her hands trembled as she took it up. Clasping it around her throat...she stood in ecstasy looking at her reflection.
She asks if she can borrow this necklace, and her friend replies, "Why, of course."
Readers may wonder why Madame Forestier so readily loans Madame Loisel this "diamond necklace" because the fact that the necklace is described as "diamond" misleads the readers from perceiving it to be costume jewelry. The only reason for readers to suspect the necklace's value is the quickness with which Madame Forestier loans the item to Mathilde. This is the only hint that the necklace may not be made of real diamonds.
Perhaps another example of foreshadowing is the contrast between the delight of Mathilde Loisel when she abandons herself to enjoyment at the Ministerial Ball and her emotions afterward when she returns home.
She danced madly, wildly, drunk with pleasure, giving no thought to anything in the triumph of her beauty, the pride of her success, in a kind of happy cloud composed of all the adulation, of all the admiring glances, of all the awakened longings, of a sense of complete victory.
Once Madame Loisel returns home, however, the fulfillment of "great reception halls . . . and small, stylish, scented sitting rooms" and the pleasure of being admired "[is] all over." This is, perhaps, a subtle hint of the future misfortune. Madame Loisel soon discovers that she has lost the necklace, and the lives of the Loisels become miserable as they struggle to pay for the real diamond necklace that they purchase as a replacement for the lost one. It is not until years later that Madame Loisel learns the truth and realizes the folly of her pride after her chance meeting with Madame Forestier.