An organization that defines best practices in the ethics of scholarly publishing. If a journal isn't predatory, the publisher should be a member of COPE.
If the record says "Not currently indexed for MEDLINE", it means that the journal may be questionable (or too new to be indexed). If you click on the journal and check the details, you'll gain more information about why it is not indexed. This is a good indicator to determine legitimacy because Medline heavily vets the journals they index.
An evaluation tool that measures the number of times an article has been cited in Clarivate's Journal Citation Reports (citation metrics with data and statistics), which can help determine the credibility of a journal.
