There are two types of checking done on STaM products, text checking and veracity of letter checking.
Text checking is done to verify that the text is completely accurate: there are no extra or missing letters or words. In modern times, text checking is done via a computer scan, as this is much more accurate than the human eye.
Every STaM product must be text checked once, and once a product has been text checked once, it does not need to be text checked again.
Veracity of letter checking is to verify that every letter has the correct appearance and is indeed kosher. This includes checking that each letter is complete. Each letter must have the halachic characteristics of that letter, according to the ktav in which it is written (e.g. Beit Yosef). No letter may be cracked, separated, missing crowns, touching another letter, or pasul in any other way. This check must be done by a Sofer STaM (not the one wrote it, as a person often does not see his own mistakes) who is knowledgeable about the requirements for the letters and how they should appear.
Veracity checking should be done on all STaM products periodically. The Shulchan Arech mandates that mezuzot be checked twice in seven years or once every 3 ½ years. There is no similar mandate for tefillin, but I recommend checking them every 10-14 years. In my experience, they deteriorate over time, and something which was kosher last time it was checked is may not be kosher the next time.
When I sell a new STaM product, both types of checks have been performed, and I have corrected any errors found, so that you can be assured you are receiving a kosher product.
When you engage me to check an existing STaM product, whether mezuzot, tefillin, or a Sefer Torah, I will perform veracity checking, and correct pesulim which I find. If a large number of tagim (crowns) is missing, there will an extra charge to add them. If a p’sul is not fixable, then I will let you know that the product is not kosher and cannot be corrected.
When I am checking a Sefer Torah, I will have it text checked (scanned) as well.