you need a friendly ABAPer to write a program that reads all POs which are subject to a release procedure and per PO he has to check the change history to find the last release like explained by Prasoon.
This is certainly quicker than looping through the whole CDPOS and CDHDR table as this change history table collects all changes done by a user in a ERP system, it usually one of the biggest table in a ERP system.