Heat Soak Testing of Thermally Toughened (Tempered) glass is carried out for the purpose of minimising the risk of spontaneous fracture of the glass, due to the presence of Nickel sulphide inclusions. However, spontaneous fracture of the glass sometimes occurs, even after the manufacturer has claimed that Heat Soak Testing was carried out. Therefore, a method for checking if Heat Soak Testing was actually carried out would be a useful tool for solving disputes concerning the cause of the fracture. The Thermal Toughening process results in residual compressive stresses on the exterior surfaces, with tensile stresses in the interior. The shape of this variation of residual stress is approximately parabolic. During the Heat Soak Testing process, some annealing of the glass takes place but the rate of annealing is different in the different layers through the glass thickness, which results in a change of the shape of the stress variation curve. Theoretical calculations of the annealing rates in the various layers of the glass were carried out and this paper presents the results and proposes a new method for checking if Heat Soak Testing was carried out based on the shape of the stress variation curve measured using photoelastic methods.