We use the mathematical toolbox of the inverse scattering transform to study quantitatively the number of solitons in a far from equilibrium one-dimensional system described by the defocusing nonlinear Schrödinger equation. We show that combining two simple criteria we are able to identify the localized eigenvalues of the Lax operator, corresponding to grey solitons and distinguish them from the continuous part of the spectrum, corresponding to plane-wave excitations. We apply this to the study of spatially periodic states and evidence the opening of multiple gaps in the spectrum. Our method can be applied in principle to all physical systems described by the defocusing nonlinear Schrödinger equation and allows to identify the solitons in numerical simulations.