The best evidence in support of the conspirators in seen in the reactions that the crowd have to Caesar's holiday in Act I. In the first scene, we see people loyal to Pompey removing wreaths from the statues of Caesar and rebuking those who are praising Caesar. Later in the Act, we also learn that, though he refused the crown three times, Caesar's refusal was less and less willing each time. We also see Caesar's pride in Act III in his treatment of Calpurnia's dream and his response to the plea to return an exiled Roman. The trait of pride speaks to the claim that Caesar is ambitious.
As the audience, we should be questioning the real motives of the conspirators, however, especially Cassius. Shakespeare highlights Cassius' manipulative personalty in his interaction with Brutus, specifically in getting Brutus to join the conspiracy (Act I, scene iii and Act II, scene i). We see that, though claiming to be noble, Cassius is motivated by jealousy and ambition himself. Finally, the appearance of Caesar's ghost in Act IV should cause us to question the true motives of the conspirators, since the arrival of the ghost implies a guilty conscience.