Once LR is out of the water for any period of time there is going to be some die-off of the organisms living on/in the LR. That dead material will decompose causing the release of ammonia, etc., etc.
If it's out of the water for any log period of time you can test it in a bucket of sw as suggested. You can also cure the LR in a tub for a few weeks to let a cycle complete, and then transfer it to your tank.
If bringing if home from the lfs, find out how long they have had it in a curing tub. IF it's been there for weeks+, then bring it home wrapped in wet towels in an insulated box to prevent as much die off as possible.
If you are only adding a few pounds of new LR then you SHOULD not have any problems