The DOAJ is an index of open access journals from across the world that has established clear criteria for inclusion in their index. Inclusion in the directory guarantees a minimum threshold of quality. Not being listed in DOAJ does not equate to being predatory, but researchers should be even more diligent before submitting their work to an unlisted journal.

It is generally expected that all journals have an ISSN unless they are brand new. Based on this Cabell's post, predatory journals may have an ISSN, but many do not, so lacking an ISSN should be a red flag, prompting researchers to be wary of submitting without further verification of the journal's legitimacy.

Journals listed in Scimago have been indexed in Elsevier's Scopus database.

The University of Mississippi does not have access to Web of Science, but you can still check whether a journal is being indexed.

MEDLINE is the National Library of Medicine's (NLM) premier bibliographic database and indexes journals based on recommendations from the Literature Selection Technical Review Committee. Note: You may be familiar with PubMed, which includes MEDLINE, but also indexes many more journals without any direct oversight (i.e., journals may be more questionable, articles may be preprints)