Look for the materials on the tags. Polyester, nylon, rayon, spandex, etc are all synthetic fibres. However, clothing that is labelled as being made of natural

Look for the materials on the tags. Polyester, nylon, rayon, spandex, etc are all synthetic fibres. However, clothing that is labelled as being made of natural fibre ("100% cotton", for example), can still be stitched with synthetic thread. I am sure regulations on this vary from country to country, however.