The easiest way to tell the content of the fabric is to perform a burn test. This is the second-best way of identifying fabrics - the best is to examine the fibers under a microscope.
Before you perform the test, prepare a fireproof surface (a flat dinner plate will work) and make sure there is a container with water near by just in case.
Clip a long thin piece of the fabric, put it in the center of a plate, and burn one end of the fabric.
Cotton's fibers (made from plant) ignite easily. If the fabric is pure cotton, nothing will remain but gray ash that easily disintegrates. When ignited it burns with a steady flame and smells like burning leaves.
Linen fibers (made from plant) takes longer to ignite than cotton. The fabric closest to the ash is very brittle. Ash crumbles easily.
If the fabric is pure silk (made from protein), the fabric will be hard to ignite and will give out a smell of burned hair. The flame is not steady. The ash is easily crumbled. The flame is not as easily extinguished as cotton or linen.
Wool (made from protein) is harder to ignite than silk. Wool weave is looser than silk. The flame is steady but more difficult to keep burning. The smell of burning wool is like burning hair.
Polyester (polymer made from coal and petroleum) melts and burns at the same time. the burned edge will have small hard plastic globules, synthetic material melts rather than burns. The smoke from polyester is black with sweetish smell. The ash is hard.
Rayon burns much as cotton does, as it is cellulose and is hard to tell from cotton with a burn test. It burns fast and leaves crumpling ash. The burning smell is close to burning leaves.
Nylon (made from petroleum) melts and burns rapidly. It smells like burning plastic.
Acrylic (made from natural gas and petroleum) burns readily. The ash is hard. The smell is acrid and harsh.
Acetate (made from cellulose wood fibers) burns rapidly with flickering flame and cannot be easily extinguished. It drips and leaves hard ash. The smell is similar to burning wood chips.