I got the one wrong about the burn test. Never done a burn test on fabric. Have you?
I do a burn test if I'm not sure what the content of fabric is. If you light a piece of fabric and it melts and leaves a hard residue instead of ashes, it's not 100% cotton. Here are couple sites that explain how to do a burn test:
http://www.lindrix.com/fabcontent.html
http://quilting.about.com/od/fabricembellishment/a/burn_test.htm