Semiconductor lasers with strong optical feedback from a plane mirror, or a weakly dispersive diffraction grating, have a frequency noise spectrum that contains a number of noise modes at frequencies close to the modespacing of the external cavity. Experiments and simulations are presented which characterize the measurement of this frequency noise using absorption on an atomic transition. The frequency noise spectroscopy lineshapes are modelled, and characterized experimentally, as a function of the strength of the optical feedback to the semiconductor laser, the detection frequency, the longitudinal mode spacing of the laser system of semiconductor laser and external cavity, the Doppler width of the atomic transition used for detection, and the order of the noise modes. The comparison of simulations and experiment indicate that the measurements behave as expected when the laser system operates in a robust single mode, as determined by optical spectral analysis. The results reinforce the utility of frequency noise spectroscopy as a means of sensitively characterizing the output of semiconductor lasers with optical feedback.