When is it OK to use other people's material without permission? The first thing to do is determine if the work is in the public domain. If it is, you're free to duplicate, distribute, derive, display, and perform publicly. If it's not in the Public Domain, then you have the option of defending your use with one of the exemptions or you must pay the piper (copyright owner) or don't use it.
How do you know if it's in Public Domain? It's in the public domain if it's in the FRIDGE of course.
Government works (U.S.)
F is for Facts. Any fact, historical, scientific, biographical, news reports are Public Domain. News broadcasts may be copyrighted but not the news itself. Facts cannot be copyrighted.
R is for Recipes. Recipes are really just an example of facts it makes the mnemonic work, otherwise we'd have fidge. But it's a good example of facts vis-à-vis creativity.
Because recipes are facts they are therefore not copyrightable, there's only one way to list the ingredients for the world's best chocolate chip cookie. Cookbooks on the other hand may be copyrighted especially the creative elements, like annotations and pictures and special arrangement of the recipes, but the recipes themselves within the cookbook are Public Domain. Recipes and the like although not protected by copyright can be protected to a degree by the intellectual property laws governing trade secrets.
I is for Ideas. Ideas cannot be copyrighted. But, applied ideas can be patented as long as they are novel, useful and non-obvious. But ideas themselves cannot be copyrighted so copy away!
D is for Dedicated. Works may be dedicated to the public domain. You will sometimes see statements to this affect on Web sites. Dedicated works are those that the copyright owner explicitly gives to the public domain as in "I grant this to the public domain" or words very much to that effect. They may also contain conditions (for instance only for nonprofit use). Like "This chart may be freely duplicated or linked to for nonprofit purposes. No permission needed. Please include web address on all reproductions of chart so recipients know where to find any updates." However, if a work is clearly in the public domain, yet it has a copyright notice on it, the notice is meaningless. Also, long elaborate copyright notices have no bearing on Fair Use.
G is for Government. All works published by officials of the U.S. government are public domain. However, private contractors sometimes write government publications, but retain the copyright and sometimes the government republishes a work that is copyrighted but got permission. The government has a wealth of information and images. This can be a real boon to webmasters. And if you're looking for pictures to put in your PowerPoint presentations search your subject in Google and limit to .gov sites.
E is for Expired. Expired works are those which have reached the limit of their copyright protection. Unfortunately this is not always easy to determine. Mainly because the copyright laws have changed over time and they are not retroactive. However, on January 1, 2003 there was a shout of joy heard round the world from archivists everywhere, because on that day a large number of unpublished works entered the Public Domain.
What about older published works? That depends mostly on what copyright law was in effect at the time of publication. If you want to know, you can use the chart below. Everything before 1923 is Public Domain unless it has been renewed. To find out if a work has been renewed, you can pay the Copyright Office $20 per hour to do a search for you, you can hire a private copyright search firm like Copyright Clearinghouse or you can conduct a search yourself at the Library of Congress Copyright Office. For more information on investigating the status of a copyrighted work see Investigating Copyright Status.
"ll terms of copyright run through the end of the calendar year in which they would otherwise expire, so a work enters the public domain on the first of the year following the expiration of its copyright term. For example, a book published on 15 March 1923 will enter the public domain on 1 January 2019, not 16 March 2018 (1923+95=2018).
Unpublished works when the death date of the author is not known may still be copyrighted, but certification from the Copyright Office that it has no record to indicate whether the person is living or died less than 70 years before is a complete defense to any action for infringement. See 17 U.S.C. § 302(e)."
For more information on when and how copyrighted materials enter the Public Domain see Circular 15a "Duration of Copyright: Provisions of the Law Dealing with the Length of Copyright Protection" and Circular 15t "Extension of Copyright Terms" from the Library of Congress Copyright Office. This Fact Sheet (SL 15) will be helpful also "New terms for copyright Protection".
For additional help in your understanding of Copyright terms and conditions for works published and unpublished outside of the United States see the Public Domain Chart published by Cornell Copyright Information Center.