Function XPLMCheckMenuItemState 

pub unsafe extern "C" fn XPLMCheckMenuItemState(
    inMenu: XPLMMenuID,
    index: c_int,
    outCheck: *mut XPLMMenuCheck,
)

XPLMCheckMenuItemState

This routine returns whether a menu item is checked or not. A menu item’s check mark may be on or off, or a menu may not have an icon at all.