There are several ways in which an EMP can check if a session has stopped.
The most certain way for an EMP to know if a session has stopped, is if you have received the ChargeDetailRecord (CDR). When the CPO pushes the CDR, you as an EMP can see that the session has ended. The CPO has 90 days to push the CDRs (in case errors occur). Usually it is pushed immediately after the session has been concluded. The CPO must always send the CDR as described by the protocol - also in cases when no energy has been consumed.