Probe beams are scanned with respect to waveguide substrates to generate optical harmonics. Detection of the optical harmonic radiation is used to image waveguide cores, claddings, or other structures such as electrodes. The detected optical radiation can also be used to provide estimates of linear electrooptic coefficients, or ratios of linear electrooptic coefficients. In some cases, the poling of polymer waveguide structures is monitored during fabrication based on a second harmonic of the probe beam. In some examples, third harmonic generation is used for imaging of conductive layers.