I'm not surprised that the very pretty fabric is a poly/cotton blend. They are often gorgeous, probably because they are intended to be made into a dress, etc. I can usually tell if a fabric is a poly/cotton blend without resorting to burning. Often the frayed threads along an edge are noticeably thin threads, compared to what the frayed threads on all cotton fabric looks like. Also, since I prewash, dry and iron everything before it goes into my stash, I can tell it's a poly/cotton blend when ironing the fabric. I always iron on the hottest setting, which is too hot for polyester, so if the fabric is a poly/cotton blend, it will give off a pungent odor. I recently learned that colored polyester fabrics glow in a blacklight, so that's another way to distinguish them. Cotton will only glow under a blacklight if it is white. Anything white glows.