How was it discovered that Iago was behind the plot to destroy Desdemona, Cassio, and Othello? Explain please.
All is revealed in the final act, and, essentially, in the final scene. When Othello asks Emilia about Desdemona cheating on him, she punctures the story Iago told, and everything unravels from there, until Iago stabs his wife (kind of a sign of guilt) and tries to escape. Later, he confesses all.