“Prove you’re deaf” would be a pretty rude thing to say, but you also don’t want to hire someone who’s lying about a disability. Presumably you’d do some kind of vetting before an in-person interview, and certainly before a hire.
Anyway in Germany I bet there’s a Taubenausweis (Gehöhrlosigkeitsbescheinigung?) or other form of official status marker, and the employer would expect you to show it to HR.
