We propose a procedure to characterize the intrinsic nonlinearities of bulk acoustic wave resonators by performing one-port measurements of the second harmonic and second-order intermodulation spurious signals. Closed-form expressions have been derived that relate the nonlinear stiffened elasticity with experimental observables. These formulas are valid in a wide range of frequencies around the resonance and have been verified with nonlinear circuit simulations. The measurement setup and its effects are also discussed in our approach. Measurements of a set of aluminum nitride-based devices from several manufacturers yield consistent model parameters and allow us to obtain a coefficient of the nonlinear stiffened elasticity intrinsic to this piezoelectric material.