
Back in 1987, when I first jumped on the speed piecing / machine quilting bandwagon, a local shop sold yard goods. You've seen them I'm sure, places where folds of fabric are stacked in colorful array, but content is sometimes questionable. The sales clerk assured me the white fabric was indeed 100% cotton. After assemply, attempts at machine quilting verified, that no, it was not. Blends will bunch and pucker and fight you all the way. To say I was disappointed would be an understatement. This is one quilt that I do wrap furniture in when moving.
How do you protect yourself from such a mishap? One way is to buy a small swatch or fat eighth if possible. Turn your iron all the way up, lay a piece of freezer paper on your ironing board, and press the fabric on top of the freezer paper. If it melts in any way, it's a blend. Cotton and flax are plant fibers, they will burn to an ash if you put a match to them. Anything with man made fiber will melt or glop up in some fashion.
I have learned to rely more on the "hand" or feel of a fabric. Some finishes can fool you though, especially Moda brand, which have that slick glossy coating. When tearing fabric to straighten the grain, blend fabric will resist and be harder to tear. Blends receive dye differently and often have a shinier surface. Also, check the bolt end, but don't rely on it. Fabric can be rewound on different bolts or mislabeled. By educating yourself on brand names (which can be found on the fabric), fabric lines, and thread content, you'll learn to make wise choices and avoid investing in unusable fabric.
Until then, keep stitchin' . . .