Http://serebii.net/pokedex-bw/445.shtml
take a good hard look: the belly is more orange than red, and it is overally a slightly more rugged purple.
And if you think you have found a wild shiny (is garchomp wild in B/W?) and you're not sure....always catch it! I did this waaaay back in emerald when I encountered a solrock but did not see the sparkles so I didn't know....I caught it and it was indeed shiny!