The Prophet ﷺ would not know where one surah ended and another began until "In the name of Allah, the Most Gracious, the Most Merciful" [Fatihah 1:1] was revealed to him.In another narration, when it was revealed, he knew that the previous surah had concluded and that another surah had begun.
كان النبي ﷺ لا يعرف فصل السورة حتى تنزل عليه {بسم الله الرحمن الرحيم} وفي رواية: فإذا نزلت عرف أن السورة قد ختمت، واستقبلت-أو ابتدئت- سورة أخرى.
Abu Dawud (788), Hakim (1/231), and Bayhaqi (2/42) all narrated it through Sufyan bin Uyayna, from Amr bin Dinar, from Saeed bin Jubayr, from Ibn Abbas, who mentioned it. Its isnad is sahih. The second narration is found in Bazzar, Kashf Astar (2187), with this same isnad.