In this letter optical and electron paramagnetic resonance (EPR) techniques are used to investigate the effect of stoichiometry changes on the site distribution of ions in crystals with (or without) MgO or ZnO codoping. In congruent crystals codoped with MgO (ZnO) the ions begin to enter into the site for concentrations higher than 4.5% (4.7%). Below this MgO (ZnO) threshold ions enter only into the sites. However, for nominally stoichiometric crystals, the threshold concentration is much lower (between 0 and 0.2% in our crystals). The use of ions as an optical and paramagnetic probe to evaluate the grade of stoichiometry is considered.