In this current life when images etched in memory come to surface through enlightenment, then one can realize what he or she was in the previous life. The insight that one gets at that moment is a manifestation of the previous life etched in our memory. At that moment events of the previous life become apparent. This occurs for not merely the immediate previous life, the realization can take place for as many as nine previous lives. There are some instances when an ascetic has been able to know and see his life, thousands of years back. However, normally one can perceive one to nine previous lives only.
Some people are able to recall their previous births, some people come to know through those who have the knowledge of the science of birth, death and rebirth; who they are, who they were and from which direction they came.
When one is constantly thinking about - "Who am I? Where did I come from? Where will I go?", then one is compelled to look for answers to such questions. One becomes curious to know - Who was I in the past? Where have I come from? Where will I be reborn? To satisfy the curiosity of such people Bhagvan Mahavir proved by practice that knowledge of previous birth is possible (Acaranga sutra, first chapter).