Yes, we as anglers, be it fly or spin casting, have a responsibility to exercise appropriate care. That's simple commonsense. And then, there are people like my sister, who I told three times to not stand directly behind me while I was casting, chose to ignore me, and got hooked under the chin not two minutes later.
The only person at fault for hooking your sister is you! If you were throwing daggers of discharging a firearm would you have carried on with her in the line of fire? You were probably lucky it was your sister. I imagine if you told a stranger "don't to stand there or I might stick this hook in you" and you then went and did it you might end up in court.
As for your question about how a layperson would know if it is safe to pass; the simple answer is to speak up and ask to pass. At the same time, even someone who has never held a rod in his or her life can understand that if the angler (whose focus may be on the water or the rod, and may not be aware of passers-by) is moving the rod, then there may be -- just may be -- a potential of being struck with the rod or the hook.
As has already been said the layperson has no idea what the fly caster is about to do. It never crosses their mind that the fly is going behind the angler or indeed that someone would be stupid enough to throw a sharp object behind them without looking in a public place.