Fisrt only buy from where you trust, ( at least here in south florida where there are many stores, some suck, somee are aweesome, and a few have been caught harvesting illeagally. Plus of course cost, we have every where formm3.99 to10.99 lb, Onlne isnt a bad way togo, but you dont get to hand pick.
Second if rock is in a salt tank with it probably is alive, or will be once cured
Third look close, some critters are visiable, worms, sponges,plants/algea, specifically coraline
Fourth, smell it if it has a kind of rotten egg smell its not cured yet.
If your stll not sure, buy a small bit from an Online source, This sight has nice stuff, and buy the piesces you like and cure it yourself. its easy to do, just need time, a large enough container for all the rock of cousre saltwater, (even stuff from water changes will work, maybe even better) and a good power head.