You cannot clearly identify Open Educational Resources (OER) by the fact that they are free. While Open Educational Resources are always free, not all free resources are Open Educational Resources. Free resources may only be accessible for free for a limited period of time. OERs are characterised by the fact that the rights attached to them allow teachers to modify, add to and share them without first obtaining specific permission from the rights holders. A special permission is not necessary because OER are typically distributed under an open (Creative Commons) licence. With this, the author retains the copyright, but others learn how they can use a resource.

The licences of Open Educational Resources are characterised by the fact that they grant all users the following rights in perpetuity referred to as the 5 Rs: Retain, Revise, Remix, Reuse, Redistribute):

1. Retain: The right to make, to possess and to control copies of the content (for instance downloading).
2. Revise: The right to use the content in different contexts (as in a learning group, on a website, in a video).
3. Remix: The right to edit, adapt, change or modify the content (for instance to translate content into another language).
4. Reuse: The right to combine original or edited content with other open content and create something new from it (like including images and music in a video).
5. Redistribute: The right to share copies of a piece of content with others, in the original or in one’s own revisions (for example giving a copy to a friend or posting it online).

Therefore, in order to clearly identify and distinguish Open Educational Resources from merely free educational resources, you should check the copyrights or intellectual property licences attached to the materials. Public domain works automatically fall into the OER category, as do works shared under a less restrictive Creative Commons licence.