It is a definitive test. Wipe away your doubts, once for all. Take a few threads of the fabric and burn them.
Protein (Silk/Wool, Cashmere, Alpaca etc):
* Burns slowly and shrinks or curls away from the flame. Will not stay lit after flame is removed. Very little smoke is produced but it smells like burnt hair or feathers. Ash is a gritty powder or a dark brittle, easily crushable bead.