  We present a method to detect the presence and depth of dark solitons within
repulsive one-dimensional harmonically trapped Bose-Einstein condensates. For a
system with one soliton, we prove that the shift of the density in Fourier
space directly maps onto the depth of the soliton. For multi-soliton systems,
combining our spectral method with established imaging techniques, the
character of the solitons present in the condensate can be determined. We
verify that the detection of solitons by the spectral shift works in the
presence of waves induced by density engineering methods. Finally we discuss
implications for vortex detection in three dimensional Bose-Einstein
condensates.
