  Recent observations by the PAMELA and AMS-02 telescopes have uncovered an
anomalous rise in the positron fraction at energies above 10~GeV. One possible
explanation for this excess is the production of primary electron/positron
pairs through electromagnetic cascades in pulsar magnetospheres. This process
results in a high multiplicity of electron/positron pairs within the
wind-termination shock of pulsar wind nebula (PWN). A consequence of this
scenario is that no circular polarization should be observed within PWN, since
the contributions from electrons and positrons exactly cancel. Here we note
that current radio instruments are capable of setting meaningful limits on the
circular polarization of synchrotron radiation in PWNs, which observationally
test the model for pulsar production of the local positron excess. The
observation of a PWN with detectable circular polarization would cast strong
doubt on pulsar interpretations of the positron excess, while observations
setting strong limits on the circular polarization of PWN would lend credence
to these models. Finally, we indicate which pulsar wind nebulae are likely to
provide the best targets for observational tests of the AMS-02 excess.
