We demonstrate the excitation of second harmonic radiation of noncentrosymmetric nanoparticles dispersed on a planar optical waveguide by the evanescent field of the guided mode. Polarization imaging reveals information on the orientation of the crystal axis of individual nanoparticles. Interference patterns generated from adjacent particles at the second harmonic frequency are--to the authors knowledge--observed for the first time. The actual form of the interference pattern is explained on the basis of a dipole radiation model, taking into account the nanoparticles' orientation, surface effects, and the characteristics of the imaging optics.