There is a bag (1) that has at least one lateral gusset (4) and at least one port (8), a source (10) of pressurized gas, elements (11) for measuring the pressure of the gas; —there are two restraining plates and spacer strips. The bag is placed flat between the two restraining plates (2a, 2b). At least one spacer strip (15a, 15b) is inserted between the two small walls of each gusset in order to prevent these walls from resting on one another. The port is connected to the gas source and the pressure measuring elements. The gas is sent under pressure into the bag, the expansion of the bag being limited by the restraining plates; then, the pressure drop in the bag is compared to a predefined threshold.