I go by how the fabric feels to me, is it thin? how's the weave? I don't want anything too thin, I want my quilts to be USED.. Also, how's the pattern applied? if it's just printed on top, alot of times I'll pass.. my experience is those fade faster than others; even in a scrap quilt, I want some consistancy; and I use the back sides of fabrics quite a bit in my quilts... oh yeah, it has to be 100 % cotton.