My mom used to tell me to hold the fabric up to the light and look at it. She said "if you could read the newspaper through it, it's not good enough to put in your clothing or quilts."
I find a lot of that kind of fabric in the stores that are not quilt shops.
The other tip-off is if it feels like it's been stiffened, it usually has. And once it's washed it just becomes awful.
But I DO agree with the others who say to buy the best you can afford. No one should not quilt just because they can't afford quilt shop prices. However I'd go to thrift shops and buy a bunch of inexpensive clothing that can be cut up and used. To this day I still do that when I see a dress or shirt that would look lovely in a quilt. I simply always make sure it's all cotton. And an awful lot of those things are.
8-)