It's unfortunate that there isn't, and really cannot be, an accreditation for SEO -- the industry move too fast, and the techniques are in flux, it's just the nature of things.
But I look at it the way the way I look at mechanics: There are a lot of great ones out there, and a lot of slimy ones out there. The slimy ones make a bad name for all their peers, despite the many people doing hard, important, quality work.
There are many wonderful SEOs (and Mechanics) out there, but if you're ever in doubt, just ask to speak to some of their clients. You'll know the good SEOs because their clients will be ambiguous in praise, with clear definable results they can discuss.
