if there is no label and you are trying to determine the fiber content, wrap it several times around your fingers to get a small hank. Use tongs and work over the sink with a small bowl of water. Light it with a match, then extinguish. Poly or poly blend will melt leaving a hard bead or bump. Cotton will just produce ash.
Also, if you are not sure of how old it is, make sure you do a break test.