The most common way to test ground resistance is with a fall of potential tester some times called a earth megger.
It has 3 leads one goes to the ground rod under test,the second lead goes to a driven test electrode 60'from the rod under test and the 3rd driven test electrode is at 120'from the rod under test.All three of the points should be in a streight line. Now what happens is that a voltage is passed between the rod under test and the rod at 120'. The rod at 60' measures the voltage drop between the 60'point and the rod under test.The unit works on null balance an you adjust the 4 knobs until the zero center meter reads zero.The figure at the 12:00 on the knobs is the resistance of the ground.
A word of caution a new ground rod will have a higher resistance than one that has been in the ground for a couple of days.Let the earth heal around the rod before testing you will get a better reading.
