Handling heat flux deposition from ELMs (Edge Localized Modes) is an essential issue for a next step fusion device. In JET, a high time resolution infrared system is used to measure temperature distribution and evolution on the divertor target plates. The quantitative and time resolved estimate of the heat flux is computed with the 2D code THEODOR constrained by the IR temperature measurements. This procedure can be complicated by the presence of a thin surface layer of low thermal conductivity and low adherence, resulting in a higher surface temperature for a given heat flux. This paper presents an experimental technique, based on a power step L-mode discharge and a flexible 1D model calculation, to assess the surface layer properties onto the JET divertor tiles.