Answer:
TEXTURE
Synthetic silk is unrealistically smooth and straight while natural silk is not so even in its texture: there will always be certain imperfections indicating the handmade nature of the material.
The soft and elastic nature of silk allows for one easy test you can make: just squeeze the fabric in your hand tightly and see if it leaves any crease marks. Natural silk should not have them while the synthetic one will have clearly visible creases.
GLOSS
To spot real silk vs. fake silk, you need to see how the fabric reacts to light. Natural silk will produce a slight gloss - if you place it in the sunlight, you may even see the silk shine and sparkle a bit. Artificial silk, on the other hand, is rather dull and does not shine in the light.
TACTILE SENSATION
Natural silk is always gentle and soft to the touch - an instantly recognizable and unique sensation, like the fabric is slowly melting in your hands. Synthetic silk does not feel anything like this - it feels like a regular soft cloth.
There’s another great way how to tell if the fabric is silk or polyester - it’s the temperature. Artificial silk will leave a cold sensation on your skin while the real silk quickly assumes your body temperature and feels natural.
RESISTANCE TO TEAR
Let’s check synthetic silk vs. natural silk tensile strength. Synthetic silk can be easily torn into pieces while natural silk is really tough to tear as its threads are very strong.
One more test you can perform - pull out a thread from the cloth and try to fluff it up. If you’re dealing with natural silk, the thread will remain unchanged while a synthetic silk thread will actually become fluffy.
PRICE
Arguably the most obvious way to tell the real from the fake. Natural, quality silk is a rather expensive material and it simply cannot be sold for the same price as cotton or linen.