First, look at the lining of the tie. The lining is to make tying the tie easier and to prevent it from wrinkling. Generally, there are two kind of linings for silk tie: 100% silk linning, 100% polyester lining.
Second, feel the fabric of the tie. Silk is smooth; other fabrics imitate silk tend to feel brittle. A quality tie will be made from three pieces of fabric, while a cheaper one will be made from only two.
Third, check the manual work: A hand-stitched finishing keeps a much better shape than a machine-made one.
Fourth, check the density. high density can be pulled to gather the tie together. This helps to maintain the shape of the tie.