I am not specialized with this, what I learnt is, to know which hour it falls, you need to know the coordinates of the birth place, The time of birth is taken with the first breath taken after birth. The surroundings that were present during her first breath, like bells hearing, rain, flash news ...... This will help the astrologer to get to know.
One more method that is used is taking the time before 11 AM and after 11 Am and co-relating with the things that has taken place. Like clashes and combinations that takes place with the hour pillar and the lower part of the body. The hour pillar- how it has effected the mother/father/uncle/aunt/siblings/grand parents. Their is an incident .. after the birth of a child the father had an promotion, increase in salary and travelled overseas. Hear promotion/ raise in salary is good, at the same time father is away from wife and child for a year. There are humpty of things with the hour pillar. So I request you to recollect all those good and bad things that has taken place.
Hope the seniors can help us on how to get more information on this.