I'll pass on what I learned in a class I took.
No matter what the fabric, touch it, feel it, rub it, look closely at it, you can feel the quality and you can see the weave and those are your best indicators. And if it feels rough when you are handling it, chances are no amount of water softener or dryer sheets will soften it up.
That said, what I also learned is that as fabric is being produced, i.e. passing through the machine, it becomes more "refined" as it progresses. When a run is done, the yardage at the front of the line (lower quality) goes to places like Joanns. The fabric at the end of the line (higher quality) goes to your LQS, or higher end fabric stores.
I have the luxury of living in an area where there are a lot of fabric places to choose from but I generally rely on the good ole touchy feelie method.