This issue confuses many collectors. According to the TB HQ website, silver and gold beads were first stamped from January 2006 and glass beads were first stamped from May 2008. Therefore, many genuine beads (all?) made before that time are not stamped.
Most genuine Trollbeads products will be stamped with the following:
Silver/Glass: 925S LAA
Gold beads: 750 LAA
Gold chains: 585 LAA
To confuse matters further, there are different stamps for different countries on precious metal.