Non-thermal electron cyclotron emission (ECE) spectra are measured by comparison of low-field side and high-field side observed ECE. The asymmetry in the respective spectra can be ascribed to the existence of non-thermal populations. Optical-depth effects are directly determined from electron cyclotron absorption measurements. Measurements are compared with theoretical ECE spectra, obtained on the basis of calculations with a quasi-linear bounce-averaged Fokker-Planck simulation code, including a simple model for radial diffusion. This yields information on the non-thermal population responsible for the measured non-thermal spectra. The good agreement between experiment and simulation corroborates the quasi-linear bounce-averaged Fokker-Planck model.