  It is one of the most challenging tasks at the Large Hadron Collider and at a
future Linear Collider not only to observe physics beyond the Standard Model,
but to clearly identify the underlying new physics model. In this paper we
concentrate on the distinction between two different supersymmetric models, the
MSSM and the NMSSM, as they can lead to similar low energy spectra. The NMSSM
adds a singlet superfield to the MSSM particle spectrum and simplifies
embedding a SM-like Higgs candidate with the measured mass of about 125.5 GeV.
In parts of the parameter space the Higgs sector itself does not provide
sufficient indications for the underlying model. We show that exploring the
gaugino/higgsino sectors could provide a meaningful way to distinguish the two
models. Assuming that only the lightest chargino and neutralino masses and
polarized cross sections $e^+e^-\to \tilde{\chi}^0_i\tilde{\chi}^0_j$,
$\tilde{\chi}^+_i\tilde{\chi}^-_j$ are accessible at the linear collider, we
reconstruct the fundamental MSSM parameters $M_1$, $M_2$, $\mu$, $\tan\beta$
and study whether a unique model distinction is possible based on this
restricted information. Depending on the singlino admixture in the lightest
neutralino states, as well as their higgsino or gaugino nature, we define
several classes of scenarios and study the prospects of experimental
differentiation.
