Q. How do I know if something is public domain?
The term public domain refers to works in which copyright protection no longer exists. This may be because the copyright has expired, or because the copyright owner has placed the work in the public domain.
For example, although the copyright in Shakespeare's plays expired long ago, many of the published editions of his plays contain added original materials (such as footnotes and prefaces) that are copyright protected because the creators of these footnotes and prefaces still retain the copyright for their work.
This creates a new copyright in the added footnotes and prefaces, but not in the underlying text of the original work by Shakespeare in which the copyright had expired.
Learn more from this University of Alberta Public Domain Flowchart.
Note: This answer comes from the Saskatchewan Polytechnic Copyright website, and provides educational information, not legal advice.