Before the final battle, there will be a dialogue choice you have to make. To view the True Ending, go through both of them until you are returned to the menu. On Persona 3's side you will also have to progress through an extra scene that links to the previous choice that were made. To determine which scenes have not been completed, look for the "Clear!" sign that appears on them. A yellow sign indicates that you have missed something; a red sign indicates everything has been done.