Burn test
A simple home check for the fibre in a saree: burn a loose thread and look at how it burns, smells and what it leaves.
Also: flame test, thread burn test
Take a loose thread from the edge of the cloth, hold it with tweezers, and light the tip over a plate, away from the saree. Natural silk burns slowly, curls away from the flame, smells like burnt hair, and leaves a black ash that crushes to powder between the fingers. Cotton burns fast like paper and leaves soft grey ash. Polyester and nylon melt, shrink away, smell chemical and leave a hard bead. The test is rough, and a blend or a zari thread can confuse it, so it is a clue and not a proof. A Silk Mark is better evidence.