How did Duryodhana find out that Karna was the eldest of the Pandavas?

How did Duryodhana find out that Karna was the eldest of the Pandavas? He never found out. If he had known that Karna was Kunti's son, he would never have dared to wage war against the Pandavas in the first place. All of Duryodhana's hope for Victory rested on Karna’s strength. Dhritarashtra says to Sanjaya I have repeatedly heard Duryodhana speak in this vein. ‘Karna is powerful and brave. He wields a firm bow and has conquered exhaustion. O king! If I have Vasushena as my aide, even the gods will not be able to withstand me in battle, not to speak of the sons of Pandu, who have lost their spirits and their endeavour. On seeing this Karna defeated in the battle, like a snake that has lost its poison and has run away, what did Duryodhana say? 1087 (110), Jayadratha Vadha parva, Mahabharata. Lord Krishna says to Arjuna O Arjuna! Whether it was the attempt to burn down your mother with her sons in the night, or whatever Suyodhana attempted towards you in the course of the gambling match, the evil-souled Karna was the root of all that. Suyodhana always thought that he would be saved by Karna and angrily tried to seize me too. O one who grants honours! It is the firm belief of that Indra among men, Dhritarashtra’s son, that Karna will certainly defeat all the Parthas in battle. O Kounteya! Though Dhritarashtra’s son knew about your strength, he found pleasure in a conflict with you because he depended on Karna. Karna has always said, “I will defeat the assembled Parthas, Vasudeva and the kings in the great battle.” 1201 (51), Karna Vadha parva, Mahabharata. Duryodhana knew that Bhishma and Drona sympathized with Pandavas and were devotees of Krishna. Kripa and Ashwatthama (till 15th day) were pro-pandava. Karna was the only great warrior Duryodhana could trust to bring him Victory. If Duryodhana had learnt that Karna was Kunti's son, that he had vowed to spare 4 sons of Kunti, that he had told Krishna that Yudhishtira is more worthy to be king etc, then Duryodhana would never have the audacity to face the Pandavas in battle.