Ammonia 0 is the foolproof way. In the past i've always smelled the rock all over. If you smell and thing rotten then its curing still. I have bought a lb or two here and there and always smelled it before putting it in my tanks. If it smelled i didn't get it.
It will be cured when your amonia is back at 0.. and not a moment before hand...
Keep it circulated, heated to about 82-84, and change the water every two-three days...
You may even run an airline into the curing container...