Toward the middle of Act 5, Scene 3, Albany learns from a Gentleman that Goneril is dead — as is Regan, who was poisoned by Goneril. The exchange goes:
O, she’s dead!
Who dead? speak, man.
Your lady, sir, your lady: and her sister
By her is poisoned; she hath confess'd it.
Goneril’s poisoning of her sister might come as a surprise — perhaps more to a reader of the play than to an audience member of a staged production.
But the scene contains textual evidence of Goneril’s conspiracy prior to the above revelation, in the line