Proactive and nondisruptive channel probing is investigated for the measurement of channel power, amplifier channel gain, and optical signal-to-noise ratio (OSNR) without disrupting established wavelength-division multiplexed channels. Using a nondisruptive probe signal over a three-span link, wavelength switching-induced power excursions are predicted to be within +/- 0.2 dB, and OSNR as high as 20 dB is accurately measured with a maximum of 0.5 dB error.