There are two standard ways to measure. The meters that do the measurement usually support both. First method involves two mobile shallow ground electrodes you

There are two standard ways to measure. The meters that do the measurement usually support both. First method involves two mobile shallow ground electrodes you place 20m away from your ground rod (so electrodes are 40m apart and your ground rod in the middle). I'm not sure how exactly it is possible to measure this when the quality of ground those two extra electrodes is unknown, but it works. The meters cost few thousands $ so I assume it is not trivial. The second method involves an already known good ground - like for example one supplied by the electricity company. Here they usually bury galvanised steel thick tape along their ground cables. That tape is hundreds of meters long and underground so it is assumed to be a really good ground. It is only used to ground outside metal bits of the power delivery equipment that are likely to be touched by people and it is separate from the ground connection supplied in the cable itself although one is supposed to connect it to the delivered ground when connecting the house cable to the electricity delivery point. Then every house is supposed to have at least two lightning protection grounds and an extra "electricity supply" ground that is again connected with the ground in the supply cable. Please bear in mind I'm no electrician and all of the above is based on me observing electricians at work when they were connecting my house, my workshop and working on lightning protection for both. I may be mistaken on some details.