SGCP uses the very same methods used by other gold miners, juniors or majors. You take sample/s from different sites in the mine, test it, and then extrapolate assay results.
Did a gold miner count its hundred of thousands of "proven" gold ounces of reserve? Absolutely, not. The keyword is extrapolation.
SGCP pays $80k/year for the geologist to do the site prospecting.