It's a common mistake to test something other than the grounding system and then conclude the grounding system is good, regardless of its actual (and unknown) condition. That mistake, in turn, leads to inadequate maintenance of the grounding system.
IEEE-142, the Green Book, differentiates between "static grounding" and "lightning grounding," by which it means bonding in the former case and earthing in the latter. Bonding, which is addressed in detail in the NEC Art. 250, Part V, isn't the concern here. Our concern is the earth connection, because you need it for lightning protection. To understand the maintenance requirements, it seems logical to look to the lightning protection standards.
The two most widely accepted lightning protection standards are NFPA 780 and LPI-175, produced by the National Fire Protection Association and the Lightning Protection Institute, respectively. These publications provide requirements for the design and installation of a lightning protection system, of which the grounding system is a part.
In its section on inspection and maintenance, LPI-175 recommends an annual visual inspection, augmented every 5 yr with a thorough inspection conducted by a qualified engineer or lightning protection installer. That "thorough inspection" includes ground testing. With methods that involve driving test rods into the soil (e.g., fall of potential), your test results are meaningless unless you disconnect the neutral to conduct the test. Although the clamp-on ground tester solves this problem in most situations, you still have to know what you’re doing to conduct a valid test.