I'd say the main difference between being prepared, and being paranoid is this; Being prepared is having the equipment you need to survive, possibly including a shelter, radiation pills, food, medical, and so on.. Being paranoid is sitting in your shelter surrounded by the stuff, thinking its gonna happen any minute now..
In all seriousness though, paranoia is taking what could be perfectly justifiable preparations and then adding an unrealistic and unreasonable element to it. Its being overly cautious or afraid without iminence of danger, but just the idea of it.
Most folks who would look at your reasonable preparation (if they are privy to it) and say you are paranoid, are gonna be the ones sitting on top of their house waiting for the gummint to come save them. Thats if they are lucky enough to live to do that.