Detect if a model has changed before calling save in Django

I have a database model that is being updated based on changes in remote data (via an HTML scraper).

I want to maintain a field called changed - a timestamp denoting when the last time that model's values changed from what they were previously (note that this is different from auto_now as these fields are updated every time a model's save method is called).

Here is my question:

In a model's save method, is there a straightforward way to detect if a model instance's current values are different from the values in the database? Or, are there any alternative methods to easily maintain a changed timestamp?