Premixed laminar flame speed determination is a crucial point since its values are used for the sizing of every combustion systems. In order to measure this parameter, an isochoric combustion method can be used. It consists in measuring the pressure (and possibly other parameters) for a spherical expanding flame. This method allows to get flame speed data for a large scope of important pressures and temperatures supposing an isentropic compression. However, the entirety of the flame propagation process cannot be used to compute the flame speed as heat losses will start to appear as the flame come close the wall, making the isentropic compression assumption invalid. In order to precisely determine when significative heat losses occur, a criterion based on the evolution of the flame preheat zone thickness is described in this paper. The evaluation of this new method is performed using Direct Numerical Simulations for different mixtures (methane with various diluents) at different equivalence ratios and thermodynamic conditions. Finally, the criterion is compared to already existing methods, showing a relatively good accuracy to describe wall heat losses effect on the flame dynamics at high pressure.