There are four common ways that works arrive in the public domain:
Look for items with a Creative Commons license. View each license for specific rules for using, sharing, and giving attribution.
Creative Commons copyright licenses and tools give creators a standardized way to grant copyright permissions to their creative work.
It is important to view the specific rules outlined in the specific license for each item. For example, in many cases, you may be able to use without permission, but you still have to give attribution (i.e. cite the source).
"OER are teaching, learning, and research resources that reside in the public domain or have been released under an intellectual property license that permits their free use and re-purposing by others." The William and Flora Hewlett Foundation