grounding measurement methods
You could go with the DIN EN 61557 norm. Part 5 defines measurement requirements and probably also setups for measuring earth resistance in already built structures:


  Electrical safety in low voltage distribution systems up to 1000 V
  a.c. and 1500 V d.c. - Equipment for testing, measuring or monitoring
  of protective measures - Part 5: Resistance to earth


If you are not from the E.U. the IEC equivalent should be found in IEC 61557-5.