Another excellent guide to carpet quality is the density of the pile—the closer the tufts are together, the better.  In comparing samples of the same fiber type, note how close the individual loops or tufts are to each other.  Press down on the pile with one finger and see how easy or difficult it is to penetrate to the backing.  Bend the carpet as it would be bending over a tread of a stair and compare the amount of backing material exposed.  Better carpet will have more closely spaced tufts and will therefore be more difficult to penetrate.