There are three ways you can check the copyright. First way—there will be a copyright notice. You can check for this sign – . Or you will see the word "Copyright" with the first publication date and the copyright owner's name.
Second way—the mandated time will have a limit, and it applies to all copyrights. Copyright that expires becomes public domain. And this rule applies to all works published in the United States prior to 1923.
Third way—some copyrighted work owners choose to withdraw their rights. So that they can use the website "Creative Commons" to put their work in the public domain. And the "CC0" mark is used to identify an image that has been dedicated to the public domain. In that case, you can look for this line—'Copyright and related rights waived via CC0.'