Burn it.

If you have ever checked the care labels on your clothes, you can for example see that one of your sweaters contains 100% cotton or a mix of other fibers. During my sessions in the laboratory we also did burning tests on our products. Simple observations such as smell and how the products burn were able to help us figure out what fibers our products may contain. From the start, the clothes we were given were totally ribbet for labels, so we had to figure out everything ourselves. 

This is Maria's bra. While it is burning, it is also melting and dripping, and when it is done burning it sets hard. This is typical for polyester. 

Here you are able to see the residue of a pair of burned, red panties. The fabric has melted and become hard, as well as the sample above.

The pants behave differently. When I sat a sample on fire, it was rapid and bright, and you can see the afterglow. This is the behavior of cotton.