Django: how can I tell if the post_save signal triggers on a new object?

Have a look at docs: https://docs.djangoproject.com/en/stable/ref/signals/#post-save

There is a created named argument which will be set to True if it's a new object.

As Docs stated and @seler pointed out, but with an example:

def keep_track_save(sender, instance, created, **kwargs):
    action = 'save' if created else 'update'
    save_duplicate((instance.id, instance.__class__.__name__, action))

post_save.connect(keep_track_save, sender=Group)