Items that are in the public domain can be used freely in your work however you want. This includes items published before 1923, documents produced by the federal government and publications from most states.
The Copyright Genie will walk you through the steps to determine if a work is in copyright and, if it is, when it will enter the public domain.
Another way to determine if an item is in the public domain, start with the Digital Copyright Slider. You will need to know the date of publication and if it was published with a copyright symbol ©.
Copyright Term and the Public Domain in the U.S. - Reference Table gives more context for determining the copyright status of a work, such as the length of the copyright term and the copyright status of works published or created outside the United States.
If the item was published between 1923 and 1963 with the © symbol, you will also need to check and see if the copyright was renewed. Book renewals can be found using the quick search in Stanford’s Copyright Renewal Database.
Renewal records for other media formats can be found in digital copies of the Catalog of Copyright Entries made available online through the Internet Archive and Online Books. When looking in these scanned copies, remember that copyright renewal had to take place in the 28th year, so look at records for 27 to 29 years after the copyright date.