Drona knew it for certain, the Pritha's son would never tell a lie, not even for the sake of getting the earth of the three worlds.
स्थिरा बुद्धिर्हि द्रोणस्य न पार्थो वक्ष्यतेऽनृतम्। त्रयाणामपि लोकानामैश्वर्यार्थे कथञ्चन॥