Mademoiselle de Chartres is a sheltered heiress, sixteen years old, whose mother has brought her to the court of Henri II to seek a husband with good financial and social prospects. When old jealousies against a kinsman spark intrigues against the young ingénue, the best marriage prospects withdraw. The young woman follows her mother's recommendation and accepts the overtures of a middling suitor, the Prince de Clèves. After the wedding, she meets the dashing Duke de Nemours. The two fall in love, yet do nothing to pursue their affections, limiting their contact to an occasional visit in the now-Princess of Clèves's salon. The duke becomes enmeshed in a scandal at court that leads the Princess to believe he has been unfaithful in his affections. A letter from a spurned mistress to her paramour is discovered in the dressing room at one of the estates—a letter actually written to the Princess' uncle, the Vidame de Chartres, who has also become entangled in a relationship with the Queen. He begs the Duke de Nemours to claim ownership of the letter, which ends up in the Princess' possession. The duke has to produce documents from the Vidame to convince the Princess that his heart has been true. Eventually, the Prince de Clèves discerns that his wife is in love with another man. She confesses as much. He relentlessly quizzes her—indeed tricks her—until she reveals the man's identity. After he sends a servant to spy on the Duke de Nemours, the Prince de Clèves believes that his wife has been unfaithful in more than just her emotions. He becomes ill and dies (either of his illness or of a broken heart). On his deathbed, he blames the Duke de Nemours for his suffering and begs the Princess not to marry him. Now free to pursue her passions, the Princess is torn between her duty and her love. The duke pursues her more openly, but she rejects him, choosing instead to enter a convent for part of each year. After several years, the duke's love for her does finally fade, and she, still relatively young, passes away in obscurity.

Question: How does the prince de Cleves find out that the Princess is in love with another man?
Answer: he has a servant to spy on her
Alternative answer: He gets the Princess to confess.