I get big cones of thread from commercial sewing places - usually inside the cone, there will be a tag that tells what it's made of. If not, look at thread under magnifying glass, and you can usually tell if it is cotton, synthetic, or a blend. The couple times I have used serger thread, it has been very linty, and easy to tell that it was not made for sewing machine. The wooly kind would be too stretchy/springy to be much good for piecing.