When a man and a woman are married and the woman becomes pregnant, it is assumed that the man is the child's father. If the couple marry before the child is born, the man is also assumed to be the baby's father. However, when the couple marries after the baby is born - or never marries at all - establishing paternity becomes a bit more complicated. If the man is fairly sure he is not the child's father, he can prove it through paternity testing.
It will be up to the child's mother to file a paternity action against the man she claims is her baby's father. The man will be required to appear in court and if he denies that he is the father, then he will have to undergo genetic testing. Genetic testing is 99 percent accurate, with results available in about two weeks.