By taking 75As hyperfine coupling constants for AsO44- in hydrogen bonded crystals of RbH2PO4 and (NH4)H2PO4 as fingerprints of ferroelectric (FE) and antiferroelectric (AFE) phases respectively, it is shown that the variation of the coupling constant as a weighted average of Rb+ and NH4+ concentrations in Rb1-x(NH4)xH2PO4 clearly proves the existence of FE and AFE fluctuations in the mixed crystal.