The first question on many peoples minds when they feel silk for the first time is "is it real?". Luckily, it's not hard to find out.
Silk is a natural protein fibre spun by silkworms. Synthetics on the other hand, such as polyester or nylon, are fibres made through chemical synthesis of petroleum products.
The quickest and easiest way to check if a fabric is real silk is by observing its sheen. Silk fibres have a unique shape allowing them to refract light at different angles, producing a distinct shininess and luster. Synthetic fabrics do not have this property and will look dull in comparison.
Silk (left) showing its distinctive glossyness compared to polyester (right)
The effect is most pronounced on satin weaves, where the uniform threads produce a strong refraction of the incoming light. For other weaves such as silk twill, where the fibres are criss-crossed, the silk is most identifiable from it's distributed sparkling, akin to a fabric filled with thousands of tiny diamonds.
On printed fabrics, another visual test one can do to check for real silk is to examine the way the ink has interacted with the fabric. Ink will not penetrate as deeply into synthetic fabrics resulting in a very one sided print. The reverse of the fabric will retain it's mostly white appearance with the image visible but wholly unimpressive. Printed silk however will have a much more uniform appearance, due to the deeper penetration of the ink into the fibres. The fabric will still be sided, with a distinctly vibrant and clear front side, but the reverse will retain the image clearly visible, albeit with a reduced intensity.
Silk (left) showing deep ink penetration compared to polyester (right)
One thing to note is the affect that the weave will have on the reverse of the fabric, with satin weaves having a dull back regardless of fibre used, simply due to the method of weaving. In this case you want to be looking for ink penetration on the rear as opposed to glossiness. See here for the differences between fabric materials and weaves.
The last and most sure-fire way test to check if a fabric is made from natural silk or a synthetic fibre is unfortunately a destructive one, but the results are immediate and conclusive. Simply hold a flame to the very edge of the fabric and watch how the fibres react, along with the type of smoke produced. Synthetic fibres behave like many other plastics, they shrivel and melt with faint smoke and a plastic smell.
When the same test is done on silk, the silk will burn, with thick smoke and a pungent odor akin to burnt hair. There will be no shriveling of the fabric and some black ash will be produced.