A mezuzah affixed to a doorpost goes through two distinct types of inspection, and it's important not to confuse them.
The first is the pre-purchase inspection, done by an expert magiha (corrector). Its purpose is to check letter by letter that each one has the correct halachic shape, and to confirm that no two letters have stuck together — if they have, the mezuzah is invalid. This thorough inspection must be done before buying the mezuzah, which is why it's important to buy only from someone trustworthy who can confirm the mezuzah passed inspection by a certified corrector.
The second is the periodic inspection, done after the mezuzah is already affixed. Halacha requires checking it twice every seven years — in practice, every three and a half years. This involves taking down the mezuzah and checking that all the letters remain intact without cracks, and examining the parchment folds to see if the ink has faded anywhere (common due to humidity or age). If everything looks as it was, it's put back up; if any doubt arises, ask a rabbi or Sofer.
Unlike the pre-purchase inspection, this periodic check can be done by anyone on their own, without special expertise — careful attention is enough. That said, if you're unsure whether your mezuzot ever underwent that thorough pre-purchase inspection, it's best to bring them to a certified Sofer for examination.
A good practical habit: note down the date of your last inspection, so you know when the next three-and-a-half-year check is due.