Your first indications of growth will be nice, new, white shell just underneath the mantle. If you're looking for it, you'll easily be able to tell the difference between new shell and older shell. I was starting to see new growth on my crocea by the two month point. Of course, like skinz78 was saying, croceas are slow growers and the smallest of the "giant" clams. I think their max size is only around 5-6".