I think you are safe when buying ones that are precut by the manufacturer--if a vendor is cutting fabrics into "precuts" you may/may not actually get consistency. Manufacturer pre-cuts will have the manufacturer's label on it with a description.
I once bought a bundle of FQ from a show vendor--they were all short! 17 or 17.5 by 22. I was furious cause of course I didn't even look at the FQ until about 6 months later when getting ready for a mystery quilt.