Journals that are indexed have typically been subjected to scrutiny by professional committees or indexers using standardized criteria. Inclusion in well-known indexes like PubMed is a useful signal regarding a journal's legitimacy.
The following sites are good starting points for investigating journals and publishers.
For more information or to read the full text of this notice, visit: NIH Notice Number: NOT-OD-18-011
The publishing community is engaged in various efforts to help scholars identify predatory journals and create standards and codes of ethics.