bool  SymbolExist(
   const string  name,    // symbol name
   bool&   is_custom      // custom symbol property
   );

#define SYMBOL_NAME "GBPUSDn"
//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
//--- declare the custom symbol flag and check the presence of a symbol having its name specified in SYMBOL_NAME
   bool custom = false;
   bool result = SymbolExist(SYMBOL_NAME, custom);

//--- declare the default 'symbol not found' message text
   string text = StringFormat("The symbol '%s' was not found among either the standard or custom symbols.", SYMBOL_NAME);

//--- if a symbol is found, create a message text depending on which list the symbol is found in
   if(result)
     {
      //--- if this is a standard symbol
      if(!custom)
         text = StringFormat("The '%s' symbol is available on the server.", SYMBOL_NAME);
      //--- if this is a custom symbol
      else
         text = StringFormat("The symbol '%s' was found in the list of custom symbols.", SYMBOL_NAME);
     }

//--- send the message about the check result to the journal
   Print(text);
   /*
   result for standard 'GBPUSD' symbol:
   The 'GBPUSD' symbol is available on the server.

   result for custom 'GBPUSDx' symbol:
   The symbol 'GBPUSDx' was found in the list of custom symbols.

   result for missing 'GBPUSDn' symbol:
   The symbol 'GBPUSDn' was not found among either the standard or custom symbols.
   */
  }