How can you tell what sex a 12 week Australorp chicken is?

I added four chooks to the flock recently and I am starting to worry that my 2 australorps are roosters. I bought them as hens so I just put their huge size (compared to the other two.) down to a heavy breed. But now I could have sworn I have been hearing the beginings of a crow. This has happened to me before but it was easier to tell the difference between an Isa Brown hen and an Isa Brown rooster.