If you're buying silk fabric right off the spool, perhaps to have something custom made, you might be interested to know that fabric manufacturers in China are required by the government to attach fabric identification tags to their goods. Therefore, check the spool with the silk you're interested in for its tag. On it there should be a five-digit number. Reading from left to right, if the first number is a "1", then the fabric is 100% real silk.
But keep in mind fake tags are even easier to produce than artificial silk. Rely on the burn test above all else.