I agree with the others, you'll be able to tell pretty easily by bodyshape and feathering, even early on. Does your chick have ridiculous amounts of feathers on it's legs? If so, that's a cochin. If it's legs are ridiculously long, and only mildly feathered, then it's a Langshan.
Is your chick a little fluffball where you wonder if there is a bird in there somewhere? If so, Cochin. Does it look like it's trying to fly a pirate flag with it's tail? Langshan.
Also, post pics! There's some pretty good guesses here in this forum!