There are several ways to identify public domain content.

1. The work may have a CC0 (sometimes called CC Zero) mark on it. CC0 is  a "no rights reserved" mark, which enables scientists, educators, artists and other creators and owners of copyright- or database-protected content to waive those interests and thereby place their works them as completely as possible in the public domain.

2. The work may have a Public Domain Mark on it. Creative Commons developed the Public Domain Mark to enable works that are no longer restricted by copyright to be marked as such in a standard and simple way, making them easily discoverable and available to others. 

BEWARE (content hunting can be perilous!) 
Just because a piece of work has no copyright associated with it does not make it automatically a part of the public domain. Some works may be public domain in some jurisdictions, but not in others. And if you are unsure, you should always check with the owner/creator to be safe.