  Recent theoretical investigations have shown that spin currents can be
generated by passing electric currents through spin-orbit coupled mesoscopic
systems. Measuring these spin currents has however not been achieved to date.
We show how mesoscopic spin currents in lateral heterostructures can be
measured with a single-channel voltage probe. In the presence of a spin
current, the charge current $I_{\rm qpc}$ through the quantum point contact
connecting the probe is odd in an externally applied Zeeman field $B$, while it
is even in the absence of spin current. Furthermore, the zero field derivative
$\partial_B I_{\rm qpc}$ is proportional to the magnitude of the spin current,
with a proportionality coefficient that can be determined in an independent
measurement. We confirm these findings numerically.
