In William Shakespeare's "The Tragedy of Julius Caesar," Artemidorus likely knew about the plot to kill Julius Caesar through Brutus. Despite the suggestion of taking an oath (Act II, Scene I) to swear all the conspirators (Cassius, Brutus, Casca, Cinna, Trebonius, Metellus Cimber, Decius Brutus, and Caius Ligarius) into secrecy, Brutus is quick to dismiss the idea, stating: 

No, not an oath. If not the face of men,
The suffering of our souls, the waste of time–

Although it is not explicitly stated, it is speculated that due to Brutus' trusting nature and overall excitement about the plot, he is the likely culprit in disclosing the details of the conspiracy. 

Artemidorus writes Caesar a letter warning him to take heed and going as far as listing each of the conspirators by name. This detailed account could only be obtained if one of the conspirators directly informed Artemidorus. Caesar, however, dismisses the letter and the result is obviously tragic. 

In my opinion, the reason Brutus is the one likely to have told Artemidorus about the plot is based on his friendship with Caesar, and that perhaps on a subconscious level he was trying to warn him himself in the hope that Caesar would learn the truth before it was too late. This supports why Brutus was so quick to dismiss swearing an oath.