If you can take a tiny piece and put a flame to it, you can tell if it's synthetic. Synthetic melts; cotton burns to ash. They really frown on that inside stores though!!!

Since you could read the selvage, you could google it and see if you could get information that way.

Sadly fiber content seems never listed on the selvage tags.

Also cottons tends to burn smelling like paper; synthetic tends to smell sour/bitter & MAY leave a bit of a bead of residue.

Not sure what a poly/cotton blend would burn test like, but I don't think it be catastrophic for the quilt (those blends tend to be 50/50 or 65/35, guessing the cotton would be the 65).

Perhaps at an estate sale, they'd let you either clip the corner to burn or pull several threads out, go outside & burn them. Watch those finger tips!!