you can't
what you can do is get the last value using the appropriate php function, if you are using MySQL its mysql_insert_id

you can guess it will almost certainly be that +1, however in a multi user system, short of attempting to secure a table lock you cannot guarantee what the next autonumber ID will be. In any event an autonumber column should have no meaning or value outside the system itself.

just out of curiosity: why would you want to know what the next ID is going to be?