Apparatus and method contemplating an in-situ test method for a head gimbal assembly (HGA). The method includes individually exciting a pair of opposing-effect microactuators supporting the read/write head adjacent a data storage media; summing the outputs from the microactuators to derive a spectral frequency response of the HGA; determining a resonant frequency from the spectral frequency response; driving the microactuators at the resonant frequency; and varying the read/write fly height apart from the data storage media to correlate contact with a maximum response of the microactuators.