I can tell if a fabric has nylon, orlon, polyester, and/or dacron in it - not necessarily which one - but that it is not 100% cotton or rayon - when I do a burn test. I get black smoke, and/or a hard bead/ash - which stays hot much longer than cotton ash. (Ask me how I know.)
I can't tell cotton from rayon because they both usually burn with white or no smoke and leave a powdery ash.