  The microscopic nature of the hidden order state in URu2Si2 is dependent on
the low-energy configurations of the uranium ions, and there is currently no
consensus on whether it is predominantly 5f^2 or 5f^3. Here we show that
measurement of the basal-plane nonlinear susceptibility can resolve this issue;
its sign at low-temperatures is a distinguishing factor. We calculate the
linear and nonlinear susceptibilities for specific 5f^2 and 5f^3 crystal-field
schemes that are consistent with current experiment. Because of its dual
magnetic and orbital character, a \Gamma_5 magnetic non-Kramers doublet
ground-state of the U ion can be identified by $\chi_1^c(T) \propto
\chi_3^\perp(T)$ where we have determined the constant of proportionality for
URu2Si2.
