An experimental method for observing the saturation of the Poole-Frenkel (PF) effect in insulating films is developed, allowing observation of this theoretically predicted effect for the first time. This method is also applied to measure the ionization potential of Coulombic attractive traps in SiO2 films. New insights into the temperature variation of the PF effect shed light on the range of validity of the classical model of Frenkel. These methods and insights are general and can be applied to many insulating films, particularly those with leakage currents dominated by field-enhanced thermal emission.