Yeah, I'd agree that is this context where Duolingo sometimes offers fragments like that, there's no way to tell the difference aurally.
You can try listening also to the slow version, sometimes it helps. At least it helped me in this case.
You don't say 'Je suis un agent", because it is your profession, and in French(also in Dutch), there comes no article for this profession, so that's why it is 'Je suis agent' or 'il est agent'. But 'it is an agent' is translated by 'c'est un agent' for example. Hope it's clearer now.