I don't think you can! I've just started programming using the IPN API for subscriptions and my answer might not be 100% correct. However, this is how I think it works.

You will receive an IPN with the txn_type of subscr_eot at the end of a subscription. This will indicate that a subscription is ended, and then you can change your database accordingly. There are a few other txn_type variables that you also need to look for, subscr_cancel, subscr_failed, subscr_modify, subscr_payment and subscr_signup. Something to think about is that subscr_signup is not telling you that payment has been made, first when you've received the subscr_payment you know for sure that a subscription has been successfully activated. For further information look here: https://cms.paypal.com/u..._html_IPNandPDTVariables