The authors show the efficiency of local measurements with an EPR probe by considering the mixed crystals Rbx(NH4)1-xAlF4 and Fe3 probe substituted for Al3+. In pure NH4AlF4 they study the oriental order-disorder transition which occurs near 155 K. EPR lineshapes are interpreted from a Kubo-Anderson dynamic model involving random jumps between local configurations, and evidence a 2D-3D crossover in the vicinity of 160 K. In mixed crystals, the room temperature EPR spectra show a random distribution of rubidium and ammonium ions for any x, and allow a simple and accurate measurement of x. At low temperature, they study the influence of the rubidium concentration on the ordering of the ammonium sublattice for 0<x0.25. For x>0.03, new low temperature orders appear which are interpreted as being structural spin glass phases.