A simple method using the singularities of an optical resonator to determine the thermal lens of end-pumped laser crystals is reported in this letter. At certain resonator parameters the transverse structures of the laser beam become a superposition of many transverse cavity modes of equal resonance frequencies. Because the resonator parameters depend on the thermally induced lens in the active medium, the strength of the lens can be determined by means of measuring the shift of the degeneration resonator length depending on the pumping power. The method is applied to an end-pumped Nd:YAG laser. ©1997 American Institute of Physics.