If using a crankbait and you have a good feel for what the bait is doing, you should be able to feel just a slight difference between rocky bottom and shells. Also, you'll pick up small shells on your hooks. If using a jig or c-rig, again you should be able to feel a difference, plus you'll often get shells on your line just ahead of the jig or weight.
Are certain areas more productive than others - you bet. How you determine them beyond catching or not catching fish is beyond me.
the fisherman formerly known as Blu-Ranger....