How does Mark Antony come to know about Ceasar's death in Julius Ceasar?
Marc Antony finds out about the death of Caesar in Act III Scene i. Trebonius, one of the conspirators, removes Marc Antony from the scene and the rest of the conspirators assassinate Caesar. After they have killed Caesar, there is an uproar and Senators run from the Senate house. We can infer that this is when Marc Antony discovers the crime, and he sends one of his servants to Brutus and his accomplices to beg for their permission to address them and saying moreover that although he loved Caesar he will serve Brutus if Brutus promises not to punish him for his loyalty to Caesar in the past. Brutus agrees not to harm him and sends the servant back to Marc Antony to tell him to come.
You will want to examine Marc Antony's actions in the rest of this scene, as his speeches over the dead body of Caesar and his attitude to the conspirators is ambiguous at best, and it is in this scene where Marc Antony moves from being a minor character to the major character of the play.