An important aspect influencing the economic advantage of reusable launch vehicles over their disposable counterparts is the cost associated with the reuse itself. These costs are optimized through design for inspection and maintenance, as well as effective condition monitoring and predictive maintenance. In this context, the fouling of rocket cooling channels due to the thermal decomposition of hydrocarbon fuels, known as pyrolysis, is particularly challenging as cooling channels are difficult to inspect and clean. Carbon depositions resulting from the pyrolysis process act as thermal insulation, due to which wall temperatures can rise and the thermo-mechanical damage to the wall can increase. The current work proposes an approach to the monitoring of cooling channel fouling based on the sensing of decomposition products in the fuel flow. In addition to acting in real-time, this approach provides an early warning of damage that will occur, as it is based on reaction products that precede the deposition of carbon. It is inherently faster than the alternative approach of sensing increases in the wall temperature. The decomposition products are sensed by thermal conductivity gauges and, using a simplified theoretical model, the data is used to infer the mass of deposited carbon in the channel. The approach is tested experimentally for methane-based fuels on Inconel 600, Inconel 625 and Haynes 230. The model is shown to match carbon deposition mass measurements to within approximately the uncertainty of the scale used. Although these results are promising, further testing under more arduous conditions needs to be conducted in future work.