Q. How do I tell is something is in the public domain?

There are now several versions of "the public domain." A Creative Commons license ( CC0 ) allows creators to dedicate their work to the public domain and waive any and all rights to their work. The more traditional way is enough time elapses that copyright rights expire. The traditional way is basically unlikely until at least 2036 due to legislation passed in the 1980s and 1990s which extended the term of copyright protection.

For an overly detailed answer, the US Copyright office says:

The terms and conditions under which works enter the public domain are a bit complicated.

• In general, anything published more than 75 years ago is now in the public domain.
• Works published after 1978 are protected for the lifetime of the author plus 70 years.
• The laws governing works published fewer than 75 years ago but before 1978 are more complicated, although generally copyright protection extended 28 years after publication plus 47 more years if the copyright was renewed, totaling 75 years from the publication date. 

The party line is: If you are uncertain about whether or not a work is in the public domain, it is probably best to contact a lawyer or act under the assumption that it is still protected by copyright laws.