As a lay-person who reads scientific papers sometimes because they’re referenced on HN, how do I validate the credibility of a journal? Does a directory of journals exist with scoring, or is there a strategy I can use when evaluating a paper or journal to determine credibility?
Beyond that you can look a bit at the impact factor of the journal, you can generally find it by just googling for the name of the journal + "impact factor" or on the Wikipedia page of the journal. But it is kinda hard to interpret this on its own, and it does not translate directly into credibility. A high impact factor shows that articles in that journal are cited very often, and you can assume that journals with high impact factors generally have a reasonable peer review system. But that doesn't say much for a single paper, reputable journals can easily fail in individual cases.
The Pubmed check generally filters out most of the journals that would just publish anything without real review, so it's the most useful filter in my opinion. I would not try to gauge more meaning than that from the journal and focus on the individual article.