[Django]-Django: Can you tell if a related field has been prefetched without fetching it?

Yes, Django stores the prefetched results in the _prefetched_objects_cache attribute of the parent model instance.

So you can do something like:

instance = Parent.objects.prefetch_related('children').all()[0]

try:
    instance._prefetched_objects_cache[instance.children.prefetch_cache_name]
    # Ok, it's pefetched
    child_count = len(instance.children.all())
except (AttributeError, KeyError):
    # Not prefetched
    child_count = instance.children.count()

See the relevant use in the django source trunk or the equivalent in v1.4.9