Mozart is perceived as a genius in the film through a couple of ways. The first is that he is a child protege. His talents as a child performer are brought out during several mentions. Salieri discusses them as a point of envy, conveying to the audience that even from the earliest of ages, Mozart's talent was anything but standard. When he does arrive as a composer to Vienna, his genius is evident, but not understood. When he is able to not only play from memory but improvise to a more intricate manner, Salieri's welcome march it is a stunning moment where his genius is on display. Salieri's description of Mozart's technical acumen is in genius terms. For example, when Salieri describes Mozart's work as "remove one note and there would be diminishment, remove one phrase and the structure would fall," it brings forth to the viewer the craftsmanship and intricacy with which Mozart crafts his music. When Mozart's wife approaches Salieri with music that Mozart has created without revision or copy, all in the first draft, Salieri is overwhelmed. The audience is, as well. Consider that Mozart did not revise his work. Meaning that this scene conveys the idea that Mozart's work are all in one stroke. That is genius. In the end, Mozart is a genius because he was not understood in his time and he has become understood throughout time, while Salieri is condemned to watch his own work become obsolete. In this, Mozart's genius is effectively conveyed.
Wolfgang Mozart is perceived to be genius because the sheer volume of his compositions includes over 600 works including 41 symphonies and 27 piano concertos.