Пуководство по пхп -> mysql_query:
Only for SELECT,SHOW,EXPLAIN or DESCRIBE statements mysql_query() returns a resource identifier or FALSE if the query was not executed correctly. For other type of SQL statements, mysql_query() returns TRUE on success and FALSE on error

типа если вернул FALSE - тада была ошибка:
<?php
$result = mysql_query("SELECT my_col FROM my_tbl")
or die("Invalid query: " . mysql_error());
?>