Many of the Umbraco services publishes a 'Saved' notification (or similar). In some cases, it is beneficial to know if this entity is a brand new entity that has been persisted in the database. This is how you can determine this.
Checking if it's new
We know that if an entity is new and hasn't been persisted that it will not have an ID. Therefore we know if an entity has been newly persisted to the database by checking if its ID was changed before being persisted.
Here's the snippet of code that does that:
var dirty = (IRememberBeingDirty)entity;
var isNew = dirty.WasPropertyDirty("Id");
To check if an entity is new in the ContentSavingNotification use the following:
var isNew = entity.HasIdentity is false;
Since the IContent has not been saved yet, it's not necessary to cast it to IRememberBeingDirty. It won't have an identity if it's new, since it hasn't been committed yet.
How it works
This is all possible because of the IRememberBeingDirty interface. Indeed the name of this interface is hilarious but it describes exactly what it does. All entities implement this interface which is really handy. It tracks not only the property data that has changed because it inherits from yet another hilarious interface called ICanBeDirty. It also tracks the property data that was changed before it was committed.