How does Elyot know that Amanda's statement of the two of them being in love all over again is false?

Elyot himself is already unhappy in his marriage to Sibyl. He knows Amanda better than anyone else and, from how she is acting, realizes that she is in the same situation that he is in. Because of their earlier arguments with their respective spouses, Elyot knows that he and Amanda are still in love and to pretend otherwise is ridiculous.