How to find out whether a model's column is a foreign key?

You can use get_field_by_name on the models _meta object:

from django.db.models import ForeignKey

def get_fk_model(model, fieldname):
    """Returns None if not foreignkey, otherswise the relevant model"""
    field_object, model, direct, m2m = model._meta.get_field_by_name(fieldname)
    if not m2m and direct and isinstance(field_object, ForeignKey):
        return field_object.rel.to
    return None

Assuming you had a model class MyModel you would use this thus:

fk_model = get_fk_model(MyModel, 'fieldname')

Simple one liner to find all the relations to other models that exist in a model:

In [8]: relations = [f for f in Model._meta.get_fields() if (f.many_to_one or f.one_to_one) and f.auto_created]

Above will give a list of all the models with their relations.

Example:

In [9]: relations
Out[9]:
[<ManyToOneRel: app1.model1>,
 <ManyToOneRel: app2.model1>,
 <OneToOneRel: app1.model2>,
 <OneToOneRel: app3.model5>,
 <OneToOneRel: app5.model1>]