An experimental technique for determining the characteristics of dielectric optical waveguides is presented. The far-field pattern of a passive waveguide excited by an external source is measured and compared with a theoretical pattern. The presence of spurious light in the fax field complicates the measurement, but this difficulty can be minimized for many structures. Application to semiconductor laser structures is discussed. Determining the waveguiding properties of laser structures prior to processing can result in the early detection of faults for increased yield.