Yup.
I'm burned again.
I am really good at identifying fabrics simply by touching them; this comes from decades of handling antique textiles.
But... every now and then I get stumped. And that's when I have to set the darn things on fire!
How do I burn test for fiber content? I simply snip off a small piece of the fabric from the inside of the garment where it will not be missed - it only takes a tiny smidge.
I place the snipped piece over a small metal bowl (an old tuna can, actually), holding the fabric snip with a tweezer. I light it on fire and study how it burns. Sometimes I use a jeweler's magnifying loop to help me "read" the results.
Then I refer to my burn chart to determine the fabric content.
The chart 0000I am sharing here is from ditzyprints.com

Sometimes I just need to add a little fire to my life!