High power electron cyclotron plasma heating in toroidal traps results in the generation of non-Maxwellian electron distribution functions. Electron cyclotron emission (ECE) spectra originating from such distributions in a close vicinity of the heating frequency are investigated analytically based on a simplified kinetic approach. An increase of the ECE level due to the quasi-linear degradation of re-absorption near the heating frequency is shown to be rather sensitive to the non-thermal distortions in the bulk electron distribution function. This effect is of peculiar interest for the diagnostics of quasi-linear effects in the case of low-field-side launch of the heating rf beam when absorbed electron cyclotron resonance (ECR) power is mainly deposited to the low-energetic electron fraction.