How to test Django custom model fields?
Django itself comes with some tests for field-subclassing; the tests have their own models.py where the custom fields are used. You should get the best impression when you have a look at the actual code yourself!
Addition: To have the models defined in your test package being discovered by django you will have to add your yourapp.test package to INSTALLED_APPS. Django itself has a built-in mechanism for its own tests to be automatically discovered and added to INSTALLED_APPS.