When a work becomes available for use without permission from a copyright owner, it is said to be "in the public domain." Most works enter the public domain because their copyrights have expired. Ask the following questions to determine if a work is available for you to use without getting permission.
You should assume that every work is protected by copyright unless you can establish that it is not.
To determine whether a work is in the public domain and available for use without the author's permission, you first have to find out when it was published. Then, apply the following rules to see if the copyright has expired:
The Copyright Office will check renewal information for you at an hourly rate. (Call the Reference & Bibliography Section at 202-707-6850 or email email@example.com.) You can also hire a private copyright search firm to see if a renewal was filed.
If you want to go the least expensive route, you can conduct a renewal search yourself. The renewal records for works published from 1950 to the present are available at the Copyright Office website at http://lcweb.loc.gov/copyright. The renewal records for works published before 1950 are available in the 660 volume Catalog of Copyright Entries, which have been digitized and made publicly available online on the Internet Archive at www.archive.org/details/copyrightrecords.
You can't rely on the presence or absence of a copyright notice (©) to determine whether a work is protected by copyright, because a copyright notice is not required for works published after March 1, 1989. And even for works published before 1989, the absence of a copyright notice may not affect the validity of the copyright -- for example, if the author made diligent attempts to correct the situation.
Any work created by a U.S. government employee or officer is in the public domain provided that the work is created in that person's official capacity. This rule does not apply to works created by state and local government employees.
Millions of works have been dedicated to the public domain. This means the author or other copyright owner chooses to give up all rights in the work forever. This is often done online using a Creative Commons CC0 license (see https://creativecommons.org/publicdomain/). However, using a CC0 license is not required. A copyright owner's use of any words unequivocally dedicating a work to the public domain can suffice--for example, words such as "this work is dedicated to the public domain and may be reproduced without authorization. However, unless there is express authorization placing the work in the public domain, do not assume that the work is free to use.
Each day, people post vast quantities of creative material on the Internet. Much of this potentially qualifies for copyright protection.
Whether particular material does qualify for copyright protection depends on factors of which you would have no knowledge -- such as when the work was first published, whether the copyright in the work has been renewed, whether the work is a work made for hire, and whether the copyright owner intends to dedicate the work to the public domain.
If you want to download the material for use in your own work, be cautious. It's best to track down the author of the material and ask for permission. The only exception to this is if you want to use only a very small portion of text for educational or non-profit purposes. (For more information, see Getting Permission to Publish: Ten Tips for Webmasters.)
For help locating material you can use without permission, see The Public Domain: How to Find and Use Copyright-Free Writings, Music, Art & More, by Steve Fishman (Nolo).