I can "tell" (from most fabrics) with a piece as small as 1/4 x 1 inch.
One's neighbors may think one a bit odd sitting by a table burning snippets of fabric - - - but it's nice to know what one has to work with.
If one plans to burn quite a few pieces - it is better to do it outside. I've done it in our laundry tubs that are in the basement (they are cement - not plastic) - but the smoke smell from the synthetics gets to be a bit much.