Newbie, help needed - Compare form value to db value

I created a custom member field, i need a way of notifying the member when the custom field has changed.

This means comparing the $_POST value to the database BEFORE it is saved to the database, and if it has changed notify the user of this.

Is this possible with an extension? I have a basic extension built but i cant workout which hook would be the one to use.

~Any help greatly appreciated.