We numerically calculate the reliability with which one can optically determine the presence or absence of an individual scatterer in a randomly occupied three-dimensional array of well-localized, coherently radiating scatterers. The reliability depends on the ratio of lattice spacing to wavelength and on the numerical aperture of the imaging system. The behavior can be qualitatively understood by considering the dependence of Bragg scattering modes on lattice spacing. These results are of interest for atomic implementations of quantum information processing.