What criteria to use when evaluating an invitation to be an editor from a legitimate publisher?
I'd be wary of requests to join the editorial board of a journal that you must ask these questions. In many (legit) cases, one is asked to join the editorial board after having established a relationship with the journal. Two of my colleagues, after having been asked, remarked 'well, I'm already reviewing for them 3 times a month so why not make it official?'  If you haven't reviewed for, or published in that journal, it may not be worth your while.