Spud,
That's not what "atheist" means. Try this: your a-leprechaunism is the your non-belief in leprechauns, but you cannot categorically say that they don't exist. A-theism works the same way, and for the same reason.
If I cannot categorically say leprechauns don't exist, then I can't disbelieve in them. I can, however, have a lack of belief in them. There seem to be two definitions of atheism: actual disbelief and lack of belief.
Anyway, I'm still quite interested in your question about how to tell if one faith should be followed over another.
I gave the example of Abram and Hagar because the story seems to illustrate how someone can believe that God has said something to him but act in a way that someone else would think was not right. So the onlooker might think from observing Abram's actions that God had not spoken to him.
Thus, God may have spoken to a someone like Mohamed (eg "do not follow ungodly ways"), but Mohamed may then have acted according to his own ideas about what God said.
Last night there was a debate about ISIS on LBC. It was evident that a lot of Muslims are confused as to what jihad means, because of the way groups like ISIS teach it means literal war. Someone pointed out that these martyrs are being taught that if they die in battle for Islam, Allah will forgive past sins so they won't be punished for them in the afterlife. This is apparently motivating many criminals who want God's forgiveness to join ISIS.
The answer to your question has to be that each individual has to discern the right from the wrong.