Process Definition - How to check for an empty datetime field

I want my Process Definition to check if, when creating a new record or editing an existing record, a user has filled in a datetime field. If they have, the Process will leave the entered date. If they have not, the Process will put in a calculated date based on the Created On datetime.

How can I check whether the field is empty? Is it simply <field> before '2000-01-01 12:00 AM'? Is there a better way?