A read head is tested by measuring the thermal magnetic fluctuation noise spectrum. A non-uniformity in the magnetic field of the free layer is produced and the thermal magnetic fluctuation noise spectrum is measured, with and/or without an external magnetic field applied. A peak in the thermal magnetic fluctuation noise spectrum can be used to derive the desired dimension of the free layer, such as track width and stripe height. The resulting measurement may then be fed back into the process control for the production of the read heads if desired. Additionally, the stiffness of the free layer and the strength of the reference layer may be determined using ferromagnetic resonance peaks in the thermal magnetic fluctuation noise spectrum.