The paper presents basic procedures and methods used for determining the state of wear of high contact ratio (HCR) gear sets comparing frequency spectra and cepstra recorded throughout their lifetime tests, through vibro-acoustic diagnostics. After a study and calculation of characteristic frequencies, the authors experimentally measured the dynamical behavior of the gear sets in order to determine their frequency spectra. To gain additional information about possible damage, cepstrum was evaluated for each measurement as well. Experiments were carried out on a FZG back-to-back test gearbox, equipped with HCR test gears during their lifecycle. The frequency spectrum and cepstrum was assigned to a specific percentage of pitting occurrence. Analyzed values of amplitudes at mesh frequency components and their sidebands (as well as corresponding quefrencies) in the spectrum (and cepstrum) were compared and the state of wear was assigned to each frequency (quefrency) response. The results from lifetime tests of two gear sets indicate that by means of FFT (as well as cepstrum) analysis the incubational phase (i.e., a cavity origination under a surface in contact) of the gear fault can be determined. This is not possible using classical (e.g., visual) methods. Furthermore, it was observed that pitting and thermal scuffing are distinguishable also at higher harmonics (the fifth and even sixth), which can be also used as an indicator of gear damage. Vibro-acoustic diagnostics is a feasible nondisassembling method used for the investigation and prediction of gear failure level in HCR gear wheels. It is shown to be a reliable method capable of predicting failure earlier than a classical visual (disassembling) approach.