You basically have the same chance to get scammed regardless if it's powder or 'human grade', of course the difference is that if you get scammed with powders, the loss will be less because they are cheaper, and you can also do the melting point test.
As for checking human grade against pics, I've seen perfect fakes. You couldn't tell'em from the real thing and there was zero active ingredient in them.
Like ld50 implied, make sure you get a respected powder source and this will cut down on your chances of getting fakes, even moreso than a respected source for 'human gear'. Why? Because most of the respected sources are either getting the stuff directly from chemical companies or are second in line. Your 'human grade' source gets his anywhere he can, and even pharmacies get duped with fake gear.