How well do you maintain your equipment grounding conductors (EGC)? Do you maintain them at all? If so, then how?
First, it helps to get the terminology correct. The EGC has nothing to do with grounding. In fact, it's pointless to ground load side equipment. With the 2008 revision, the NEC made great strides toward correcting misuse of the word "grounding." But it still has to cross the finish line on that problem; thus, it prescribes "grounding" conductors that don't do grounding.
The Art. 100 definitions of "grounding" and "bonding" cut through the confusion. Based on those definitions, an EGC isn't a grounding (connect to earth) conductor. The EGC is a bonding conductor. As such, it forms a low-resistance metallic path.
EGC maintenance typically means "visual inspection." A visual inspection can find only obvious corrosion or an obvious break in the path and will miss the most likely problem. Conditions such as water ingress or harmonic current flow can, over time, result in high impedance across the connection points in an equipment bonding path.
The only way to find the problem points is to measure impedance along the entire chain of EGC, from the equipment to the next node. However, not with a digital multimeter — it simply can’t do the job. Nor should you use a ground testing instrument.
Use an impedance tester that has a "low ohms" function with AC current. How often you should test depends on how severe conditions are.