Everybody is right...it is by the "hand" or feel of the fabric. Like sheets, it is thread count and weave. My big deciding factor is color. I can work with a "thinner" fabric ( heck I will throw a fusible interfacing behind it) if the color and pattern is right. Muddy or off tone colors drive me batty.
Also shrinkage, LQS will be able to tell me that X fabric will shrink about X% which is important if you pre-wash and need a true fat quarter or are doing things like mixing flannels and cottons or cottons and Minkee. I hate it when you buy a FQ, you pre-wash it and it comes out a 16 1/2 by 19 trapezoid instead of a 18 by 22 rectangle.
That being said, I have also paid a king's ransom for a piece of fabric from a well known line and had it ravel and just be awful.
Once you are quilting awhile, you will have manufacturers that you like and you will be able to get them on sale or will be willing to bite the bullet for a special project. Like I said on the other thread, I make crafting type projects and I make quilts. Wallhangings, table runners, aprons, lap quilts...I am selective but not picky. I will buy a crappy fabric if the pattern is cute. A king size quilt I am putting 200 hours into, each fabric better be right and last.