Jocasta's last speech before her life starts to unravel is this:
"Fear? Why should man fear? His life is governed by the opreations of chance. Nothing can be clearly foreseen. The best way to live is by hit and miss, as best you can. Don't be afraid that you may marry your mother. Many a man before you, in dreams, has shared his mother's bed. But to live at ease one must attach no importance to such things."
Ironically, after dismissing the predictions once given by the Oracle, her next line (except for one) is where we know Jocasta has probably put two and two together and is now simply hoping to keep others from knowing what she knows.
The Corinthian messenger has arrived; and, at Oedipus' prodding, tells the story of Oedipus' arrival at the home of Polybus and Merope--those he thought were his parents. He and Jocasta had sent for the shephard who could corroborate this story--and who had obviously headed back to the hills once Oedipus arrived in town so he would not have to tell it--and Oedipus asks Jocasta if she remembers if the shepherd from the story is the same one they sent for. Her reply is telling, as she obviously tries to dissuade him from pursuing the matter:
"Why ask who he means? Don't pay any attention to him. Don't even think about what he had said--it makes no sense."
This is clearly the first time Jocasta's words speak of her horrific realization. When her husband/son refuses to let it drop, Jocasta knows she will not be able to live with the shame and horror. Her road to suicide begins here, though it winds back to the very beginning when she and Laius tried to thwart the predictions of the Oracles.