I thought that wasn't clearly established. I believe the assumption has been that you could call something public domain, and obviously people do so, but I don't believe it has been tested in court.
Not that I can see an obvious reason why it would be so tested, and frankly these are fuzzy memories from 15-20 years ago.
To what extent it is obvious just a simple statement that something is public domain would work depends on jurisdiction, which is why things like CC0 [1] exists.
