You need to find out how long they've had the rock before just adding it. I would recommend using the nose method if your in a hurry. If you take the rock out of water and it smells in spots really bad, then it isn't cured and it has dieoff. Obviously this is not scientific in nature, just a simple way of telling. Good luck
BTW, I would personally cure it myself anyways. Its easy to do. Cya