How can you identify that which plate of capacitor is positively charged?

FOR tESTING THE SIGN OF CHARGE ON BODY, a device called GOLD LEAF ELECTROSCOPE. When the disc of a positively charged is touched with any plate of the charge capacitor. IF the divergence of gold leaf increases, then the plate is positively charged and if the divergence in the leaf decrease then the plate of the capacitor is nagatively charged.