How does Figaro discover that Marcellina and Bartolo are his parents?

In the opera The Marriage of Figaro, Figaro discovers his parentage during a legal dispute in Act III. When the judge Don Curzio and the Count demand that Figaro either marry Marcellina or repay a loan, Figaro refuses, claiming he is of noble birth and cannot marry without his parents' consent.1

When pressed for proof, Figaro explains that he was stolen as an infant and reveals a birthmark on his arm. This mark serves as evidence that he is the long-lost son of Marcellina and Bartolo. Upon this revelation, the three embrace, and the conflict is resolved as Bartolo proposes marriage to Marcellina, and she tears up Figaro's debt.1