My member challenge this month was to fish a texas rig soft plastic. I can say to me that bumping rocks is the hardest to distinguish between that and a bite ( to me only if you are using a weight). It seems like you are getting a tap tap tap. To me, from the few fish I have caught, not only is it feel, but you have to watch your line for movement. After a while it gets easier but you have to concentrate a great deal. I actually learned more from setting the hook when I wasn't supposed to as when I was getting bites. Sorry if this doesn't help out too much, but that is the best way that I can explain it to you with my experiences.
P.S. I don't think that its a stupid question, if you are just starting to use that rig as I was, its not an easy thing to differintiate.