In general, evaluate OER in the same manner that you currently review textbooks and other sources for your courses. Examine the following:
Author's Qualifications and Affiliations
Date of Publication & Current Accuracy
Intended Audience
Documentation of Sources
Comprehensiveness
Bias
Cultural Relevance
Suitability of the Work for your Purposes
Review and Curation Process
Modularity
Restrictions/Licenses of the Work
Comments and Reviews from Other Readers