Well, there is no direct method to tell that file is empty. You can tell that file is empty when its sheet contains no data.
You can test than in multiple ways, for example:
1) $objPHPExcel->getActiveSheet()->toArray()
It will return sheet data as an array, if count = 0 - your sheet is empty.
2) $objPHPExcel->setActiveSheetIndex(0)->getHighestColumn();
and $objPHPExcel->setActiveSheetIndex(0)->getHighestRow();
If higest row = 1, and column = A - your sheet is empty.