koi actually have barbels from the time their born.
Koi look like common goldfish from the side, but their heads are bigger and shaped... differently. The mouths are also different. I'll try and get a decent shot of my poor, giant, overcrowded babies this evening to show. Koi are also a bit more... streamlined? ... they're not quite as deep in the body as Shubies are.
There's a lot more to their girth that there is to shubies. Again, they look almost exactly like common goldies in regard to conformation.
Koi also come in the tri-color so that's no help. You're more likely to find Shubunkin's with long tails that are vastly different in appearance from butterfly koi.
I'll keep trying to find some image comparisons for you.
*pics of my babies from above (not the pics I'll try and get later)*
http://shinoarnii.tripod.com/kyounichi.htmlhttp://shinoarnii.tripod.com/currentyuki.html