Operation and management of Wavelength Division Multiplexing (WDM) systems require the monitoring of optical channel frequency and power. In this paper we propose a simple and low-cost solution for tracking the frequency of WDM channels, based on the thermo-optic tuning of single or coupled-cavities Fabry-Perot silicon optical filters. The fabricated structures are single-cavity filters exhibiting, thanks to a suitable coating stack on both cavity sides, high finesse and narrow bandwidth. Moreover, the free spectral range is large compared to the channel spacing, allowing the monitoring of one carrier frequency at a time. By means of a heater we change the cavity refractive index and move the transmission peaks, thus scanning the WDM frequency set. In particular, we demonstrate the possibility to resolve up to seven 50-GHz-spaced channels, with a crosstalk of -10 dB, at wavelengths around 1550 nm. Better performances, in terms of resolvable channels and cross-talk can be obtained by using two coupled-cavities, having a common resonance peak and different free spectral ranges. The global optical transfer function of such a cascade shows only one transmission peak in a frequency range of about 30 nm, and can be thermally tuned in this range.