I have a theory that I'd like to run past folks. I am closing comments on this one, and asking Muslims who read this to drop me an email with their reaction (the address is to the right in the sidebar).
It is difficult for outsiders to get a grasp of who a "good muslim" is, when the muslim community itself seems deeply divided on the issue, takfir flying all over the place. And we already have the well-known problem of the Satanic Verses, and various arguments discussing which Sura and Hadith are to be taken within a specific context, and which are meant to be widely and generally applicable.
One theory might be the following: if God sent many prophets, all of whom said different things, particularly in the context of how people are to live, the natural logic that follows from this is one of two things:
1. God can't make up his mind about how people should live.
2. People put their own interests before God, and effectively forget what they're told over time.
#1 isn't going to get us anywhere, for reasons that should be blazingly obvious. #2, on the other hand, makes sense. So, let us run with that one. God being by definition omniscient, you have to assume that he would know ahead of time that he was going to send more than one prophet, which means that we've effectively been given a neat corrective tool by which to measure our understanding (not counting the hopeless arguments over who is and is not a false prophet)... the message should be consistent. Islam accepts that Jesus was a prophet. Jesus came to fulfill the Mosaic law, and specifically corrected those who followed the letter of the law, but not its spirit, in the process laying down two very specific and direct Commandments. It is easy to see how the two Great Commandments (1. Love God with all your heart and mind and soul. 2. Love your neighbor as yourself -- even if your neighbor is your enemy.) do not conflict in the slightest with the Ten Commandments of Mosaic Law.
Therefore, can this not also be applied to the Koran and the teachings of Mohammed? Is it or is it not possible to determine who is a true Muslim by determining the extent to which what they teach is true to what all of God's prophets have said, in stark contrast to those raving egos who change the message because they think that they know better than God?