Bhagavad Gita, Chapter 11, Verse 9

Sanjaya said: Having thus spoken, O King, Hari, the Great Lord of Yoga, showed unto the son of Prithâ, His Supreme Ishvara-Form.

This moment is described from outside as well, a real, second witness confirming what's happening.

Sanjay's narration steps in here as an independent account of the same event. Real corroboration is built into the telling itself, not left to Arjun's word alone.