Gem Snow is a snow gene that came out of Reptilian Gems. Unlike Mack Snow, Gem Snow x Gem Snow does not create Super Snows. It is considered a dominant trait rather than co-dominant.
It can be tough at times. Some of the gems I've seen have been lighter, but there are light mack snows, too. I don't know if you can really tell them apart most of the time.
Agree with Kristi. No reliable way to identify a Gem Snow besides test breeding, which I personally would not recommend due to the chance of producing more unknown Snows. It's a lot like the albino strains in that way. :\