ears are a big indicator, totally flopped ears are almost always Aussie, while its possable for a BBC to have flopped ears they are much more likley to be at least partially pricked.

anouther is the pants, the fur of an Aussies pants tend to stick out, where as a BCs is always straight down.

head shape can sometimes be an indicator, Aussies are "usually" a bit blockier with shorter muzzles, but thats not always the case.

size..Aussies are upright workers bred for cattle, this makes them a bit heavier..more substance. BCs "usually" have lighter builds.

the way they move, again an Aussie is an upright worker, they move standing tall, and tend to circle. BCs have a habit of slinking, and they dont usually circle, rather they are prone to "run out ahead..stop..turn to face you"

and sometimes putting the dogs are stock are the only way to tell lol