if you burn the edge of a piece of fabric with polyester in it the edge will become very hard...brittle, crisp...(the plastic melting) cotton edges stay (scorched) but soft.
lots of times a fabric on the bolt may (feel) different...it is usually from the added chemicals-sizing...if you pre=wash your fabrics the cottons will be back to being nice and soft...feel better than a blend...a blend the hand does not change with washing.
usually on the salvage it will say who the manufacturer is and the fiber content...98% of my fabrics say 100% cotton on the salvage....i have seen fabrics wrapped on bolt boards they do not belong on...totally different fabric stated on end of bolt from what is actually there...i always check the end and open up the fabric and look at it, making sure i am getting what i pay for.