There are a variety of rubrics you can use when assessing an OER; however, some of the key criteria you'll want to focus on include:
Source: Derivative of "Faculty Guide for Evaluating Open Education Resources" by BC Open Education Librarians, available under a Creative Commons Attribution 4.0 International license
Source: Video "How to Find and Evaluate OER" by Abbey Elder, available under a Creative Commons Attribution 4.0 International license (CC BY 4.0)