Your post made me think about it some more and I think ultimately it is just an impossible thing to quantify and by extension speculate on. It's a given that it happens, I think most can agree.
I think the litmus test is to see what stories survive the longest and are most prevalent with the least amount of tyranny. That's tyranny of the many translators/editors over time and also tyranny of organised religions enforcing their own interpretations onto devout people. For example there are many stories in the bible that I'm sure even agnostic/atheist people would recognize as great stories with a sense of real human truth and meaning, or stories that are incredibly similar put into the 'holy books' of several different religions, and I think that's a pretty good gauge to use.
That is the problem isn't it? How can an organized religion be called divinely inspired when all of their greatest teachings are merely humanistic morals? If they are truly divine, they should be talking about who we are, what our purpose is, why is there so much difference in the world in every aspect, what we should do to gain that knowledge. And none of these questions are answered by saying, "God exists, he created us" -- that doesn't actually answer anything but raises more questions.
For example, take Sermon on the Mount. Nice little lecture on a bunch of morals about helping others, turning the other cheek etc. Some laws on adultery and social issues. That's Ok, Jesus was a very saintly person based on his teachings, but why are these moral lessons even divinely inspired or godly? Common sense is enough for any of us to realize this.
Same goes for Mohammed. My personal opinion is that there are some violent verses in the Koran which have been exploited, but there's certainly some nice poetry in there. Ok, Mohammed was a bit of a poet and he talked about certain ethics and rules to be followed. But apart from repeated injunctions of "worship Allah", what is there that is even divinely inspired that we wouldn't already know.
If something is truly divine, it should talk entirely about things beyond the senses apart from merely postulating an invisible God and a meaningless creation of myriad differences that cannot be explained. A bunch of moral codes is admirable and respectable, but doesn't warrant divine status.