Based on the transverse second-harmonic generation (TSHG) effect, we demonstrate a method for in-situ modal inspection of nonlinear micro/nanowaveguides. Pumping lights are equally split and coupled into two ends of a single CdS nanobelt (NB). As pumping light counter-propagates along the NB, transverse second-harmonic (TSH) interference patterns are observed. The influence of multimode interaction on the TSHG effect is discussed in detail. Using fast Fourier transform, TSH interference patterns are analyzed, indicating the existence of at least four modes inside the NB. Experimental beat lengths are found to be in agreement with calculated results.