The activation energy of thermal fixing is determined in congruent and nearly stoichiometric lithium niobate crystals doped with manganese or iron, respectively. Three different techniques were employed: two-wave mixing, holographic scattering and DC conductivity measurements. A comparison between the three techniques is made and the possible reasons for the discrepancy in the values of the activation energy are discussed. Holographic techniques have the advantage of being contactless methods by which problems coming from electrodes effects are ignored. The holographic scattering technique is much simpler than two-wave mixing technique and gives the same results at high density of the compensating ions. At low free ions concentration it is an ideally sensitive technique to detect the possible dependence of the compensation time constant on the spatial frequency and to determine the concentration of free ions that are responsible for thermal fixing.