Various diagnostic methods are compared with respect to their abilities in determining the controlled electrode erosion in a radio-frequency (rf) plasma source. The source is cylindrical in shape with a center rf cathode made of magnesium. Due to the unequal electrode area ratio of 1:8 a high self-bias voltage develops. Sputtering from the cathode was studied by means of optical emission spectroscopy, laser-induced fluorescence (LIF), quadrupole mass spectrometry, and deposition experiments. Dependencies of the plasma parameters were measured. The magnesium density decreased exponentially with distance from the target but could be traced down to very low levels around 10(5) cm(-3) in the downstream region by the extremely sensitive LIF method. Laser excitation from the electronic ground state as well as fluorescence detection were performed on the strong 3p P-1(1)0 <-- 3s S-1(0) resonance line at 285.2 nm. For absolute calibration Rayleigh scattering from cold nitrogen gas was utilized. Sensitivity issues and detection limits are discussed.