sadly a lot of fakes are pretty close, and there's no way you can tell from a photo or even having them in hand. maybe the only way you can tell if the color is off, but you'd only be able to tell if you've seen those colors in person before, and most of the time you can't tell from people's photos because of camera/lighting/etc. avoid single colors (esp black, weird mint gums, purple/lavender, milk pink mx, etc), translucents, drunken, and probably og tri. some are obviously bad like the ones on etsy.
i pretty much stopped collecting new ones from other people unless i know where they've originally been before. i've seen a couple of people inadvertently buy fakes off ebay, so even if the person you're trading with is a good legit person, they might have a fufu one without knowing it.