What to test in a simple DRF API?

So, I'm not a test expert and sometimes, when using packages like DRF, I think what should I test on the code...

If I write custom functions for some endpoints, I understand I should test this because I've written this code and there are no tests for this... But the DRF codebase is pretty tested.

But if I'm writing a simple API that only extends ModelSerializer and ModelViewSet what should I be testing?

The keys in the JSON serialized? The relations?

What should I be testing?