code: public static function isFormula($value = '', Cell $pCell = null)
      {
          if ($pCell === null) {
              return self::REF();
          }
  
          return substr($pCell->getWorksheet()->getCell($value)->getValue(), 0, 1) === '=';
      }
docs: ISFORMULA.
  
  @param mixed $value The cell to check
  @param Cell $pCell The current cell (containing this formula)
  
  @return bool|string