How does Beethoven's friend find out that he is deaf?

Beethoven's friend, Anton Schindler, discovers the composer's deafness during a visit when he notices that Beethoven is unable to hear the music played on the piano. Schindler observes that Beethoven is increasingly reliant on conversation and gestures, and he becomes aware of the composer's struggles to engage with music in the way he once did. This realization highlights the profound impact of Beethoven's hearing loss on his life and work.