A method of identifying a fault in a system of gears in a wind turbine is provided. The method determines (204) two or more centre harmonic frequency amplitudes according to vibrations of the system of gears and determines (205) a plurality of sideband amplitudes of each of the centre harmonic frequency amplitudes. Further, the method sums (207) the centre harmonic frequency amplitudes to calculate a total centre harmonic frequency amplitude and sums (206) each of the sideband amplitudes of the centre harmonic frequency amplitudes to calculate a total sideband amplitude. The method then determines (208) a value indicative of damage incurred by the system of gears based upon the first centre harmonic frequency amplitude and the average sideband amplitude. The method may be implemented by a controller which may be used to control a wind turbine or wind park based on the value indicative of damage. The centre harmonic frequencies may be harmonic tooth mesh frequencies and the value indicative of damage may be a ratio of the total centre harmonic frequency amplitude and the total sideband amplitude or vice versa. The method may analyse the value of any ratio obtained and use the ratio values to identify, and monitor the progress of, a fault.