The moment of truth for Othello must definitely come in this play in Act V scene 2 when he finally realises what precisely has gone on and how Iago has deceived him. As Emilia reveals the various stratagems and devices her husband has used, Othello begins to contemplate how precisely he has allowed himself to be deceived, and how he is to blame for his own jealousy and taking matters into his own hands and killing Desdemona on the flimsy evidence he has seen. Note how he responds in the following speech:
Whip me, ye devils,
From the possession of this heavenly sight!
Blow me about in winds, roast me in sulphur,
Wash me in steep-down gulfs of liquid fire!
Othello can't believe the truth and his first response is to wish incredible violence and pain upon himself for the murder he has visited upon his chaste wife. The moment of truth is revealed through the character of Emilia, and it suddenly brings with it an epiphany when Othello is forced to confront his own jealousy and how it played into the hands of Iago. He has no one to blame for Desdemona's death but himself.