Do a burn test – I know that sounds dramatic but we aren’t suggesting you set your stash alight!!
In essence, cotton fibres ignite as the flame from your match draws near
Synthetic fibres curl away from the heat and tend to ‘melt’.
Take a few fibres; put into flameproof container with sides maybe, and apply a small flame from a long handled match maybe, and watch what happens.
Remember – always test in safe surroundings and away from children (who might get the wrong idea!)