How do you know if an article is in the public domain?

Answer 1

Because notification is not required for copyright protection, individuals and organizations who wish to donate their works to the public domain tend to make that clear in writing to avoid confusion. For example, the Consumer Product Safety Division article on Halloween safety has this notice:

CPSC documents are in the public domain; a CPSC document may be reproduced without change in part or whole by an individual or organization without permission.