The quality of the tuning and matching of an NMR probe on a Bruker NMR spectrometer can be monitored by using the wobble (or "wobb") routine in the TOPSPIN software. This routine sweeps the frequency using low power and displays a plot corresponding roughly to the absorbance vs frequency for the probe electronics. If the probe has very high sensitivity (eg: a cryoprobe) and contains a sample rich in protons (eg: water) then one is able to observe the proton spectrum in the frequency swept wobble curve. This is demonstrated in the figure below which shows wobble curves for a 600 MHz cryoprobe containing a sample of water.
Thank you to Stan Woodman of Bruker Canada for pointing this phenomenon out to me.