$WOOCS->current_currency

This is not a function - this is an important variable in the plugin main class WOOCS, and it's always equal to the current selected currency which actual on the front end of the site

global $WOOCS;
echo $WOOCS->current_currency;
//will print for example 'USD'

Shortcodes, functions, actions and filters reference for FOX WooCommerce Currency Switcher