In Chapter X of Book the Second of A Tale of Two Cities, Dr. Manette has an inkling of something that Charles Darnay may divulge, but begs Charles not to say more at the time that Darnay asks for Lucie's hand in marriage:
So strange was the way in which he faded into silence, and so strange his fixed look when he had ceased to speak, that Darnay felt his own hand turn cold in the hand that slowly released and dropped it.
Then, interestingly, the reader learns who Charles Darnay really is through the conversation of Madame Defarge in Chapter XVI of Book the Second. Madame Defarge speaks with her husband,
Yes, Miss Manette is going to be married. But not to an Englishman; to one who, like herself, is French by birth. And speaking of Gaspard (ah, poor Gaspard! It was cruel, cruel!), it is a curious thing that she is going to marry the nephew of Monsieur the Marquis, for whom Gaspard was exalted to that height of so many feet; in other words, the present Marquis. But he lives unknown in England, he is no marquis there; he is Mr. Charles Darnay. D’Aulnais is the name of his mother’s family.”
Charles Darnay is Charles d'Evremonde, son of the twin who so brutally caused the deaths of the brother and sister of Madame de Farge.
In Chapter XVIII of Book the Second, on the marriage day of Lucie and Charles, as Mr. Lorry attends the ceremony, Dr. Manette's room opens and he emerges with Charles Darnay. Mr. Lorry notices that Manette is deathly pale--unlike his appearance when he went in with Charles. Although he is composed, Mr. Lorry detects "the old air of avoidance and dread" has passed over him, "like a cold wind." So, it is obvious that Charles has revealed what Madame Defarge has already told her husband in Chapter XVI.