Mezuzahs should be checked by a sofer (certified scribe) twice every seven years to see if they have been affected by adverse weather conditions, or by folding (which can cause cracks in the letters), or if any other defect has occurred. It is preferable to check mezuzahs which are on the outside of a building more often, due to their vulnerability to inclement weather.
In addition, the infiltration of fraudulent mezuzahs in the marketplace present further cause for regular checking to determine the authenticity of the mezuzahs in your possession.
The checking of mezuzahs has been revolutionized by technological developments over the last several years. Mezuzahs can now be scanned by a computer for textual errors.
Since no sofer, however diligent, is infallible, computer scanning has the advantage that no word or letter will ever be overlooked.
Utilization of the computer scanning method has brought serious instances of fraud to light. In one instance, for example, the system identified several mezuzahs as being exactly identical. This is impossible with handwritten parchments. Investigation revealed that the source was merely reproducing mezuzahs on parchment using a raised-ink printing process. In another instance, the system called attention to the fact that previously scanned mezuzahs with missing words or letters had been resubmitted with those errors corrected. Such a mezuzah is invalid, as it was written "out of order."
For this reason, in addition to having the mezuzah checked by a scribe, ask the scribe if he has the option to check it also via scanning the mezuzah into the computer.
There is an ancient custom practiced by people diligent in mitzvot to have their mezuzahs checked once a year, during the Jewish month of Elul, the month prior to the High Holidays. The Lubavitcher Rebbe, Rabbi Menachem Mendel Schneerson, of righteous memory, would encourage all to have their mezuzahs checked during this auspicious month.