For me, its hard to describe how without having both breeds in front of me and pointint out differences. But don't let me fool you -- even I can mistake them at times, especially from photos because sometimes photos are taken at bad angles.

BC's tails are usually "drop tails", whereas Aussies' tails are usually parallel with their body. Their coats are usually different, too, the way the hair lays. Their ear sets and eyes and muzzles; the Aussie's muzzle is USUALLY shorter in conformation dogs.

Conformation its easy to pick apart and tell differences, but when you get into the working dogs its a whole different ballpark since they aren't bred for looks!

It just takes practice

EDIT: Also, as others have pointed out; when my two aussies' hair gets wet, the hair "crimps", and right after it dries it is VERY curly around their back end!!