“A ‘kosher’ mezuzah is hand-written on genuine parchment… A specially trained scribe, known as a sofer, carefully writes the words using special black ink and a quill pen. The letters must be written according to halacha (Jewish Law), and every letter and word must be correct. Any mistakes or missing letters invalidate the entire parchment.
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. It is for this reason that one should buy a mezuzah from a God-fearing person.”