The 'scrunch test', this is not 100% to be certain but a good guide non the less, pure cotton does crease a lot when scrunched up and held tightly in the hand, Rayon does this too but Rayon is usually a little thinner to the feel, than cotton. Poly/cotton does not scrunch up so much, it bounces back a little better, also Poly/cottons usually look the same on the back as the front, where as cottons are usually printed on the surface and on the back you can see the white of the fabric in places. These tests are a good guide when you can't 'burn down the shop' and after a while you can tell the difference quite well. The 'burn test' is what they use in the textile industry to be 100% sure, my Dad was a textile chemist and this is the method they were taught to use.
Gal