It is not possible to know if a Mezuzah is kosher just by looking at it, since part of its being kosher has to do with the scribe who wrote it, his knowledge and his thoughts at the time of writing the Mezuzah. Mezuzah requires a human hand and a clean Jewish mind. (Mezuzah is one of those few things that cannot be made by machines or be mass produced in China!). It is for this reason that one should buy a Mezuzah exclusively from a trustful God-fearing person.
A professional scribe is usually certified by “Vaad Mishmeret Stam”.
FYI see this interesting article: http://www.shemayisrael.co.il/orgs/stam/guide2.htm