As you get more experienced you will be able to see the difference. Take a piece of the fabric you like and hold it up with your hand behind it. Compare how much you can " see" your hand through the fraying fabric and to the better fabric. If the fabric seems thin and see through, it isn't the best quality.
You can also mark off a 1/2 square on both fabrics and count the number of threads in the square to compare. Lower thread count is flimsier fabric.