You can roll up a ball of the htread and do a burn test.
•Polyester: Has a somewhat sweet chemical odor. The residue is initially a hard cream-colored bead that becomes darker. Flames gives off black smoke.
•Cotton: Burns, but does not melt. It has the odor of burning paper,
•leaves, or wood. The residue is a fine, feathery, gray ash.
•Hemp: Same as cotton
•Linen: Same as cotton
•Ramie : Same as cotton
•Rayon : Same as cotton
•Silk: Burns, but does not melt. It shrinks from the flame. It has the odor of charred meat. The residue is a black, hollow irregular bead that can be easily to a gritty black powder. It is self-extinguishing, i.e., it burns itself out.
•Wool, and other Protein Fibers: Burns, but does not melt. It shrinks from the flame. It has a strong odor of burning hair. The residue is a black, hollow irregular bead that can be easily crushed into a gritty black powder. It is self-extinguishing, i.e., it burns itself out.
I remember doing burn test on fibers when I was in school for Marketing many many years ago.