As efficient a QuerySet.__contains__ as can be mustered.

gistfile1.py

def instance_in_qs(instance_or_pk, qs):
    if qs._result_cache is None:
        if qs.query.can_filter():
            pk = getattr(instance_or_pk, 'pk', instance_or_pk)
            return qs.filter(pk=pk).exists()
        else:
            objs = list(qs)
    else:
        objs = qs._result_cache
    
    if hasattr(instance_or_pk, 'pk'):
        return instance_or_pk in objs
    else:
        get_pk = lambda o: getattr(o, 'pk')
        return instance_or_pk in map(get_pk, objs)