  We show that partial transposition for pure and mixed two-particle states in
a discrete $N$-dimensional Hilbert space is equivalent to a change in sign of a
"momentum-like" variable of one of the particles in the Wigner function for the
state. This generalizes a result obtained for continuous-variable systems to
the discrete-variable system case. We show that, in principle, quantum
mechanics allows measuring the expectation value of an observable in a
partially transposed state, in spite of the fact that the latter may not be a
physical state. We illustrate this result with the example of an "isotropic
state", which is dependent on a parameter $r$, and an operator whose variance
becomes negative for the partially transposed state for certain values of $r$;
for such $r$, the original states are entangled.
