Try this: Do this over a sink so if you drop a burning piece it won't take your house with it. Take a small piece of the fabric and hold a match to it to just singe the edge. All cotton (plant material) fabric will turn to ash when you rub the blackened part between your fingers. Polyester/nylon/etc will leave a more solid, melted residue. Test it first on two pieces of fabric that you know the content of so you can spot the difference.