Methodologies are reported that allow the identification of extended defects on MgO(100) surfaces. The techniques used include low-energy electron diffraction (LEED), metastable impact electron spectroscopy (MIES), D2O and CO temperature-programmed desorption (TPD), and MIES of adsorbed Xe. LEED and MIES data for MgO(100), TPD spectra of D2O and CO, and MIES spectra for adsorbed Xe are significantly altered as a function of the density of the extended defects. NO is shown to be an ineffective probe molecule for titration of defect sites on MgO(100).