How to grade Tefillin
Consult a reliable Rabbi

There are several parameters that dictate the grade of a Tefillin to be less or more "Mehudar"
(higher quality):

also see: How to put on Tefillin - Tefillin

• The scroll
  
  The better the writing of the Tefillin scroll or the leather on which the scroll is written, the better the Tefillin.
   
• Quality control (Hagahah)
  
  The level of quality control (Hagahah) the scroll goes through before it is considered Kosher.
   
• The boxes (Batim) leather
  
  The box of the Tefillin could be defined as Tefillin Dakot (thin) which is made of sheep leather, or Tefillin Gasot (thick) which is made of Ox or cow leather. Tefillin Gasot is considered to be higher in quality.
  
  Boxes come in small to very large
   
• The boxes (Batim) structure
  
  The boxes that contain the scrolls may be constructed from one or more parts. If the box is made as one part, it is considered best, and is caleld in Hebrew "Or Echad" (one leather).
   
• The Straps
  Retzuot - Or Tachton
  Retzuot - Or Elyon
  Retzuot - Avodat Yad