During their life span, imaging spectrometers are likely to be affected by deviations in spectral performances. Such fluctuations are mainly due to vibrations and temperature/pressure changes at the moment of launch or aging of the instrument. Prior to taking the spectrometer to the laboratory for a time- consuming re-characterization and re-calibration, it is good practice to monitor its spectral performance in- flight. For the Airborne Prism Experiment (APEX) spectrometer, this can be achieved by means of an onboard In-Flight Characterization (IFC) facility. IFC data are acquired at closed shutter with a stable input signal coming from a 75 W Quartz Tungsten Halogen (QTH) lamp. A filter wheel is interposed in the optical path leading to the detector; the spectral filters mounted on the wheel are characterized by a number of narrow spectral features. In this paper the development and tuning process of an algorithm to be used for the spectral stability monitoring of APEX is presented. The study is based on simulated IFC data and aims at identifying a spectrum-matching technique to be included in the final algorithm. In this context four spectrum-matching methods are tested in a varying range of simulated measurement conditions. We found that the methods employing the correlation coefficient and the RMSD as merit functions are more suitable and robust approaches for the estimation of the wavelength shift.