Cheri,

The easiest way to tell the difference between natural fibers like cotton and mad-made fibers like poly is to do a simple burn test. Cut a little sliver of each. Burn one at a time. You can use tweezers so not to burn yourself. The cotton will slowly burn and turn to ash while the poly (whether it be 100% poly or a blend) will melt. You'll definately see a difference in the burning between your samples. The poly will also smell like burning plastic.