According to Feathersite, here is a quote
"Australorps differ from Black Jersey Giants in that the legs of the Jersey Giant are black (some tending toward willow) and the underparts of the feet are yellow. In Lorps, the legs are black, shading to dark slate in adults, but the underparts of the feet are pinkish white. This works with good show stock, but hatchery stock is more variable and you can't ever be sure unless the birds grow to the size of Giants. "
I started using legbands. Adolescent Australorp pullets are impossible (for me anyway) to tell from same age black Penedesenca pullets. Luckily when mature the Penes get white earlobes.
I keep the black Jersey Giants in a separate pen.
Another thing is that good quality Jersey Giants should differ because of their size. Quality stock should be the biggest birds in your flock.