This article is focused on the diagnostic technique used in the Thermal Engines Laboratory of the University of Valladolid (Spain), to perform real time calculation, monitoring and recording of gasification physical variables, only from gas composition analysis and some other measurements such as local temperatures and gas flow rate. By assuming some simplifying hypothesis it is possible to calculate the main gasification parameters such as biomass consumption, air/fuel ratio, thermal efficiency, and generated thermal power. The aim of this work is to explain the technique implemented and to give an idea of the wide range of applications that it can have, underlining its simplicity, its reliability and the low costs of the equipment needed.