Short answer
The quickest trade test is a burn test on a loose thread: real silk burns slowly, smells like burning hair and leaves soft, crushable ash, while polyester melts into a hard bead with a chemical smell. For high-value orders, use a laboratory test.
Ask the supplier for written fibre composition first; reputable traders will tell you. If you want to verify:
Accurate fibre content matters for customs duty (silk is HS 5007; synthetic filament fabric is HS 5407) and for labelling laws in most importing countries.
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