The method is based on placing a stainless steel or glass vessel in an oil bath or oven, adding a sufficient quantity of the adhesive to the vessel, melting the sample, measuring the viscosity at the operating temperature and the softening point, repeating these operations at regular time intervals of between 4 h and 6 h, until the time for stopping the test according to pre-determined criteria is reached, and observing and recording, at the time that each measurement is taken, the following phenomena: skin formation, fumes, phase separation, gelation, change in colour.