How can anyone know that the 10 commandments weren't just chicken scratch on some slate slabs Moses made?

Most of the people were illiterate, so they couldn't read it. The priests who kept it were led by Aaron, Moses' brother. Those who might have been able to read the things, were not allowed to see them because they were not members of the "priestly" bloodline. The bible paints a dirty picture of both of them, so why not fake some "God Given" commandments? A murderer, liar and thief isn't the best source for holy laws.