Characterization of stray light at component level is now a well-established domain, with various techniques and excellent noise floors. The cheracterization of stray light at system level is more delicate, as the final equipment may not allow the implementation of a dedicated instrumentation for the stray light measurement. For the LISA space mission we have developped a method that applies for the characterization of coherent stray light with excellent noise floor and which lends itself to the characterization of any kind of coherent stray light in any type of complex system. It is based on the Frequency Modulated Continuous Wave (FMCW) approach, but is of general application. It allows to retrieve both the fractionnal amplitude of stray light, and the stray light optical path length with respect to nominal light, and hence, identify the offending component.