Wealthy families could afford to have magics (divine or arcane) around the mother-to-be to ward off any evil presence. Thus, logically, over time the wealthy would tend to mostly have Good or Neutral souls. The poor, however, would be let open to whatever fate had in store for them--or actually, since Evil souls are being deflected from the wealthy, there logically could be a higher percentage of Evil souls coming to them. Thus the wealthy classes would have a factual basis (or additional excuse) to consider themselves intrinsically fit to rule over the lower classes. Naturally, parents unable to provide protection would be concerned about the alignment of their child, possibly even while it was still in the womb (or egg or whatever). Since paladins, and some others can detect the presence of Evil there would be a certain demand for their services.
And in some places this determination process would even be mandatory. A Lawful Neutral nation might simply have all Evil offspring killed at birth of avoid future problems. A Lawful Good nation might establish protected pregnancy places, probably in temples or monasteries, where mothers could stay full time during their pregnancy. Or the nation might round up the Evil-souled children and have them brought up in royal orphanages where every attempt to instill Good behavior would be made. After they were old enough to leave the orphanage they might even remain wards of the state for life, working and living under constant supervision in case their inner nature came to the fore. Both of these socio-political scenarios have lots of interesting consequences which make for gritty story-telling and building intense character backgrounds. And if the players' characters become parents themselves at some point then there's really dramatic tension in the offing.
A Chaotic society, however, might forbid any interference in the process of the embedding of the soul. Or all decisions might be left up to the conscience of individual parents. Or Evil newborns could be left exposed in the wilderness (or set adrift, as in the biblical story of Moses) to let fate take its course. Or upon reaching the age of maturity people would undergo a determination ritual. Persons of confirmed Evil disposition could be given certain supplies, equipment, and coin and sent out into exile to face whatever the gods had in store for them. Again, all of these make for good story drama and character design.
Now, I'm sure some people will complain about how these types of treatment would violate people's human rights, etc. Well, the idea of human rights is a modern one and pre-modern/traditional societies don't really have those modern types of legal rights. Those societies usually draw clear lines of what's allowed and what isn't and that's that. Here are some examples from the "Blue Laws" of the Colony of Connecticut in the 1650's:
- No food or lodging shall be afforded to a Quaker, Adamite, or other Heretic.
- If any person turns Quaker, he shall be banished, and not suffered to return but upon pain of death.
- No Priest shall abide in this Dominion: he shall be banished, and suffer death on his return. Priests may be seized by any one without a warrant. [I believe they are referring to Catholic priests here.]
- When it appears that an accused has confederates, and he refuses to discover them, he may be racked.
- A debtor in prison, swearing he has no estate, shall be let out and sold, to make satisfaction.
- Every male shall have his hair cut round according to a cap.
- A drunkard shall have a master appointed by the selectmen, who are to debar him from the liberty of buying and selling.
- No one shall run on the Sabbath day, or walk in his garden or elsewhere, except reverently to and from meeting.