Get name of primary field of Django model

In Django, every model has a pseudo attribute pk, that points to the field that is declared as primary key.

class TestModel(models.Model):
    payload = models.Charfield(max_length=200)

In this model, the pk attribute would point to the implicit id field, that is generated if no field is declared to be the primary.

class CustomPK(models.Model)
    primary = models.CharField(max_length=100, primary=True)
    payload = models.Charfield(max_length=200)

In this model, the pk attribute would point to the explicit defined primary key field primary

So my question is, how can I get the name of the field, that is the primary key?