Are you talking the 60s and 70s polyester double-knits?
Some of them have very little stretch. And sometimes the stretch is in only one direction.
Give them a tug and see what they do.
You could burn a tiny scrap and see what it does. If it puts out black smoke, it has poly (or nylon or dacron) in it.
If poly, it is extremely durable - it if isn't burned - Makes fantastic utility/everyday/let the kids play on them quilts.