Disconnect the "P" leads from both mags. Remember that both mags are now "HOT". Do NOT turn the prop.
Check with an ohmmeter between the "P" leads and ground.
In "both" position, neither P lead should be grounded.
In "left" position, only the right P lead should be grounded.
In the "right" position, only the left P lead should be grounded.
In the "off" position, both P leads should be grounded.