I can help you out, but to be honest it is almost a futile measurement especially in an urban environment. There are two methods primarily used.
The most accurate method (sort of) is what is called
3-Point Dead Fall of Potential test using a Meggar type high voltage resistance meter. . It involves driving two short ground rod into the dirt at specified distances from the unit under test. However there are three requirements for it to be semi-accurate which is almost impossible to meet.
The length must be 10 times the length of the GES UUT. So if that building has say a 100 foot diameter ground ring you are looking at 1000 feet of cable for P1, and 620 feet for P2.
There cannot be any metallic structure under the ground crossing the path of test field like water and gas pipes.
The test must be performed multiple time from different directions to get a weighted average.
So as you can see it is almost impossible for such a large system. Doable for say a 2-rod system, but a ring, not worth the effort.
OK I do a lot of this for a living in the Telecom sector and I can tell you how to do it in 5 minutes or less. Get your hands on a Clamp-On Ground Rod Tester like a
AEMC Model 3731 Ground Tester. I will even sell you one for $1200
Only trick to using it is clamping it around the correct cable so you do not measure a Wire LOOP. Secondly if there is significant return current flowing in the GEC will require turning-off the main breaker, but the Service Neutral must remain connected. If the facility is wired up correctly and has a Grounded Circuit Conductor service you clamp it around either MBJ or GEC that leaves the service disconnect point going to the first electrode. It is not highly accurate but more than good enough to verify if a Ground System meets a spec. It always reads higher than actual impedance. So if the spec calls for 5-Ohms and the meter reads 5 or less Ohms it passes. If you use it be sure to follow the instructions and understand what it tells you.