1 Samuel 20:30, 33

Then Saul’s anger burned against Jonathan and he said to him, “You son of a wayward, rebellious woman! Do I not know that you have chosen the son of Jesse [over me] to your own shame, and to the shame of your mother’s nakedness?

Then Saul hurled his spear at him to strike him down, so Jonathan knew [without any doubt] that his father had decided to put David to death.