The Wenner method is the standard procedure for measuring the resistivity of soil. Four electrodes are driven into the ground in a straight line, all at the same spacing: current is injected through the two outer ones and voltage is read between the two inner ones.
The apparent resistivity comes from multiplying the measured resistance by 2π and by the electrode spacing. The elegance of the method lies in the fact that this spacing determines the depth explored: as a rule of thumb, the result is the average resistivity of the ground down to a depth similar to the electrode spacing.
Repeating the measurement with increasing spacings builds a resistivity profile with depth, which is what allows a decision between a shallow mesh and deep rods. It is worth taking two traverses at right angles to detect non-uniform ground, and recording moisture and season, since the values change considerably through the year.