Jocasta pretty much gives the game away when the messenger arrives to tell her and Oedipus the story of how the infant Oedipus came to arrive at the house of Polybus and Merope.
Oedipus wanted the shepherd to corroborate this story, but the shepherd—literally—took to the hills as soon as Oedipus arrived in town, as he didn't want to tell him what happened. For obvious reasons, Jocasta feels exactly the same way. The last thing she wants is for the shepherd to say his piece, thus encouraging Oedipus to investigate his origins further.
It's at this point in the drama that we realize that the penny's finally dropped for Jocasta. Why else would she explicitly dissuade Oedipus from getting to the bottom of things? Why else would she say something like this:
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.
Jocasta's never said such a thing before. Perhaps she'd previously had her suspicions, but now we know for sure that she's aware of the terrible truth, and she'll do anything she can to prevent it from getting out.