Rob, nicely cured live rock should smell good. Like a well-kept tank smells.....oceany. UNcured rock smells like dirty workboots and makes your nose revolt. Lift a piece up and SNIFF it. If it smells bad, choose another piece. Most decent stores will separate their rockwork according to it's level of curedness. New stuff is usually kept in a separate container while it cures, and a GOOD store will tell you how long it's been curing for, and what level of risk it has, give or take.
To ensure the health of your live rock when you transfer it from the store to your home, you can ask the guys to pack it in IO buckets with water and lids. This will keep the bacteria alive until you can get the stuff home and transferred into your tanks.
Ideally, when adding new rock to a tank, you should use common sense. If the only live rock available to you at the time smells a teesny bit, then you can add smaller amounts of it to larger systems and check your ammonia levels daily. Sometimes even the BEST stores can be caught in the middle of a live rock cure. Ask them. If your LFS sucks, I feel for you. But at least you can minimize the damage by making sure the rock you do buy is dealt with properly. If you really don't trust your LFS, then always put your new live rock into a large bin for a week with a skimmer and some powerheads. Test the water for ammonia. When all parameters are reading safe, you can use it in your tanks.
Buy the rock that smells the best, so you have the minimal amount of work before you can use it. And good luck
