"Are you the manager?"
Translation:Bist du der Manager?
Bist du der Geschäftsführer? is good as well in German, although you would rather say: Sind Sie der Geschäftsführer? because it is more polite and appropriate in a work-life situation where you do not know the other person.
If you went out for a beer, got to know a man and talked to him all night and two days later you visit a company and see him again lets say in a suit and you didn't know that he was 'the Geschäftsführer' then 'Bist du der Geschäftsführer?' would suit the situation :)