It would be just the shear resistance of the slab. Assume a 45 degree plane through the section from the point of loading and then calculate the area of the shear plane. Multiply this area by the s=tensile strength of the concrete to get an estimate of shear strength. See the anchor pull out section of ACI 318 for the concept. Of course this neglects and resistance from the soil below the SOG.
Hope that helps.