The activation energy is measured by measuring the reaction rate as a function of temperature and making an Arrhenius plot. A previous post on RG covered this nicely:
Note that you need to determine two Energies of Activation, Ea: Ea(with enzyme) and Ea(without enzyme), each determined using the procedure posted above. The difference between them is the Ea reduction you are looking for. The first Ea should be lower than the second Ea, if the enzyme is working as expected.