He would know that the prayer of the Messenger of Allah ﷺ had ended by the takbir.
حدثنا أحمد بن عبدة أخبرنا سفيان عن عمرو عن أبي معبد
كان يعلم انقضاء صلاة رسول الله ﷺ بالتكبير
No references are available.
Bukhari narrated it in the Book of Adhan, Chapter on Remembrance After Prayer, no. 842, and Muslim in the Book of Mosques, Chapter on Remembrance After Prayer, no. 583, from the hadith of Sufyan bin Uyayna, with this wording.