Step 1:

Locate the work's publication date and see if it is published before 1923. If it is, the work is automatically placed in public domain. Some examples in this category include:

The Household Cyclopedia - a how-to manual from 1881
The Boy Mechanic: 700 Things for Boys to Do by Popular Mechanics - illustrations and all in PDF.
Things to Make by Archibald Williams - projects in carpentry, machinery, kites, and more.
Archive.org search for "how-to - check before copying to wikiHow because not all of the information is in the public domain.
The Jewish Encyclopedia (1901-1906)
The Nuttall Enyclopedia

(Examples from Wikihow, How to Find Public Domain Materials, CC-BY)

Step 2:

Research books that were published between 1923 and Jan 1, 1964. 90% of books during this period are not copyrighted, since their copyright holders failed to extend their copyright. Review the copyright renewal database for details.

Step 3:

Determine whether the work is eligible for public domain status. If it is a work of the US government and other federal government agencies, the work may be considered to be in public domain. Some good examples:

Space Educators' Handbook
US Forest Service Fire Effects Database - contains photos and facts on many species.
NIST Dictionary of Algorithms, Data Structures, and Problems
Centers for Disease Control and Prevention
USDA Nutrient Data Laboratory
U.S. Navy - contains good information on knots.
U.S. Department of Defense - some military training books contain good how-tos on a variety of subjects.
Federal Emergency Management Agency - contains good information on preparing for natural disasters.
National Transportation Safety Board
National Oceanic and Atmospheric Administration
U.S. Geological Survey

(Examples from Wikihow, How to Find Public Domain Materials, CC-BY).

Step 4:

If none of the above cases are met, you will have to do research to determine whether the work in question is in the public domain. Please use the guidelines found in Copyright Term and the Public Domain in the United States, developed by Peter Hirtle at Cornell University. This provides an extensive guide for determining if a work is in the public domain.

Step 5:

Keep in mind that there are a number of websites that purport to curate openly licensed images and content yet aren't legitimate. It is wise to approach the site just as you would individual works. Ask yourself if the site covers all the considerations we've mentioned above for the works it shares.