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A technique for the accurate determination of the surface potential US and its evolution during irradiation, is proposed. The technique is based on detecting both backscattered (BSE) and secondary electrons (SE) in a scanning electron microscope (SEM). The (BSE+SE) spectra are measured using a compact, highly sensitive electrostatic toroidal spectrometer (ETS), specially adapted for SEM applications. The use of an ETS analyzer set in a SEM for deducing the surface potential from (SE+BSE) spectra of electron irradiated insulators is introduced here. The surface potential is determined, either from the measured maximum energy of the secondary electron peak, or from its beginning. Various dielectric materials such as MgO, Al2O3, Y2O3, mica (potassium silicate aluminum), and Teflon were studied by this technique. Experimental investigations of the beam energy and current effects on the surface potential of bare insulators are reported. The change, due to this surface potential, in some physical quantities such as the amount of BSEs and the most probable energy of their spectral distribution is also studied. The results concerning coated and grounded insulators highlight the influence of the internal electric field on the BSEs energy distribution. An account of the various benefits of using the toroidal spectrometer in the surface potential measurement is also given.