How to assess an unknown journal's quality?
When 95% is junk and the other 5% is sufficiently irrelevant that neither you or your supervisor and colleagues know about them, a safe and efficient strategy is to ignore all journals that actively request contributions.

I have also never heard of a legitimate journal using mass emailing to recrute editors (this is more a habit of author-pay open access journals that are looking to increase their market share).

Yes this strategy, if adopted broadly, would prevent some new journals from being known. The good news is: we already have way more journals than we could possibly wish for. It's good content and quality curation we're short of.