When adding serials you enter a subscription length, using the check expiration tool you can see when your subscriptions are about to expire. To use the tool click the link to 'Check expiration' on the serials menu.
In the form that appears you need to enter at least a date to search by.
In your results you will see all subscriptions that will expire before the date you entered. From there you can choose to view the subscription further or renew it in one click.